netconvert, netgenerate, and netedit share the same process of building networks which is started as soon as data are read (in netconvert) or after an internal description is generated (in netgenerate).
The process requires that at least nodes and edges are given, optionally also connections between edges and/or lanes and other optional information. Hints about how the network shall be built  whether turnarounds or highway on/offramps shall be added, traffic lights or roundabouts shall be guessed, etc. are retrieved from the options.
Below, you may find a list of all steps performed while building the network. Optional steps are in italics.
 Modifying the sets of nodes and edges
 Joining junction clusters
 Removing selfloops
 Removing isolated roads
 Removing unwished edges
 Removing empty nodes and geometry nodes
 Joining similar edges
 Splitting geometry edges
 Guessing and setting on/offramps
 Moving network to origin
 TLS position computation
 Assigning nodes to traffic lights
 Joining traffic light nodes
 Computing connections between lanes
 Computing turning directions
 Sorting nodes' edges
 Computing Approached Edges
 Computing Approaching Lanes
 Dividing of Lanes on Approached Lanes
 Appending Turnarounds
 Rechecking of lane endings
 Geometry computation
 Computing node shapes
 Computing edge shapes
 Computing rightofway and traffic light programs
 Computing tls logics
 Computing node logics
 Computing traffic light logics
 Finishing inner edges
These computation steps are performed within void NBNetBuilder::compute(OptionsCont &oc). These steps are described more detailed in the following.
Steps#
 precondition: All input is read
 precondition: Nodes have: id, cartesian coordinates, type (optional, may change)
 precondition: Edges have: valid from/tonodes, lane number, max. speed limits, allowed vehicle classes, geometry (may change), typeID, lane spread information, name (optional), priority information (optional)
#1: Modifying the sets of nodes and edges#
Purpose: Remove unwished edges/nodes from the internal representation; add geometry nodes if wished postcondition: all unwished elements (nodes, edges) are removed, geometry nodes are inserted/removed if wished, attributes as above
The following substeps are done:

joinJunctions ('
Joining junction clusters
'): build node clustersPurpose: Join network graph nodes which form a single intersection. postcondition: junction joined, attributes as above affects: removes and adds nodes, removes edges, remaps connections (tbd) state: tbd

removeSelfLoops ('
Removing selfloops
'): remove all edges where the fromnode is the same as the tonodepostcondition: Each edge is a unidirectional connection between two different nodes (ok) state: green for me, though I am not sure why lists of edges are built which are then given to remapping methods; wouldn't it be enough to check within the remapping method whether a connection exists?

joinSimilarEdges ('
Joining similar edges
'): "join" edges which connect the same nodespostcondition: No two edges with same geometry connecting same nodes exist state: yellow, as there are two comments in the code, one about a bug, a second one about a missing check for vclasses; should be revisited (why it's here, what would happen if not) comment: ? Earlier, only one road could connect same intersections; not now. Why, explain, how does it work? comment: This method uses two magic numbers to determine whether edges are "similar":
distanceThreshold = 7.;
// don't merge edges further apartlengthThreshold = 0.10;
// don't merge edges with higher relative lengthdifference

removeIsolatedRoads ('
Finding isolated roads
'): remove edges which are isolated (not part of the network)postcondition: ? not defined somewhere state: yellow; the method is pretty large, and there is no removal of edges within the tls control; how does it work? comment: Should be revisited

removeUnwishedEdges ('
Removing unwished edges
'): Removes edges not in myEdges2Keeppostcondition: not wanted edges are no longer in the internal containers (ok) state: yellow; the complete keep/remove/keeppostload  stuff should be revisited

removeUnwishedNodes ('
Removing empty nodes and geometry nodes
' or 'Removing empty nodes
'): Removes useless nodespostcondition: empty nodes, nodes with no incoming/outgoing edges, are removed (ok) postcondition: nodes which only affect the geometry of an edge are removed if wished (ok) state: yellow; should be done in two steps for a cleaner code; additionally, we had the bug that empty acosta nodes where still in the network; is this solved?

splitGeometry ('
Splitting geometry edges
'): Splits edges at geometry positionspostcondition: If wished, edges have a linegeometry only state: yellow; the naming of built nodes is not proper

Guessing on/offramps (
Guessing and setting on/offramps
)Purpose: Add missing on/offramps state: tbd
As a result, the internal representation of the read/generated network contains the sets of nodes/edges the user wanted.
#2: Moving network to origin#
Purpose: Let network coordinates start at 0,0 state: green
#3: TLS position computation#
Purpose: Let nodes know whether they are controlled by traffic lights

guessTLs ('
Assigning nodes to traffic lights
')postcondition: empty nodes, nodes with no incoming/outgoing edges, are removed (ok) state: tbd

joinTLS ('
Joining traffic light nodes
')postcondition: empty nodes, nodes with no incoming/outgoing edges, are removed (ok) state: tbd
#4: Connections Computation#
Purpose: Determine and/or assign connections over intersections between lanes
Computing turning directions#
 code: NBTurningDirectionsComputer::computeTurnDirections(...);
 postcondition: The edge knows its turnaround direction, if existing.
 reason: Turnarounds are treated differently in the following steps than other directions of driving.
 how
For each combination of an incoming and an outgoing edge, which may be connected, we compute the angle between them. If this angle is >160°, we assume that this may be a turnaround. We add 360° to this angle if both edges connect same both nodes in different direction. We store the edge pairs with angle into a list sorted decreasing by the angle. After all combinations are computed, we go through the list and set those as turnarounds which we see at first, marking both the incoming and the outgoing edge as already seen what avoids to use them twice. We warn if an edge occurs twice with an angle >360°  multiple edges connecting two same nodes.

state: yellow
the check for being connected should be omitted, here, see inlinedocumentation as usual, you may find pathological intersections
Sorting nodes' edges#
 code: NBNodesEdgesSorter::sortNodesEdges(...);
 postcondition: each node's edges (incoming, outgoing, and all) are sorted clockwise, starting at 12 o'clock. Incoming edges with same angle as the according outgoing are before the outgoing one in the list of all edges.
 reason: this order is assumed to be given in following computation steps
 how
We sort a node's edge vectors (incoming, outgoing, all), first. Then, we go through the vector of all edges and exchange an incoming and an outgoing edge if both have the same angle, and the incoming is after the outgoing (determined using "isTurningDirection").

state: yellow
unvalidated for lefthand networks the method uses the method "NBEdge::isTurningDirectionAt(...)" which is not a plain usage of the turnaround computation performed earlier
Computing node types#
 code: NBNodeTypeComputer::computeNodeTypes(...);
 postcondition: Each node has one of the following types: NODETYPE_PRIORITY_JUNCTION, NODETYPE_RIGHT_BEFORE_LEFT, NODETYPE_TRAFFIC_LIGHT, NODETYPE_NOJUNCTION(?).
 reason: We need this to compute the rightofway rules.
 how
We investigate each node by itself. We check some special cases, first:
 if the type is already set (myType != NODETYPE_UNKNOWN) > skip
 if we have only one incoming edge > set to NODETYPE_PRIORITY_JUNCTION
 if it is a simple continuation > set to NODETYPE_PRIORITY_JUNCTION
At first, we assume the intersection is a NODETYPE_RIGHT_BEFORE_LEFT intersection. We then go through all incoming edge combinations which are not opposite. As soon as we encounter one where the speed limit between both edges differs by more than 10km/h (approx.), we set the type to NODETYPE_PRIORITY_JUNCTION. This is also the case if one of the edges has a speed limit of more than 49km/h (we use this value to avoid floating point issues).

state: yellow
investigate whether NODETYPE_NOJUNCTION can be set earlier (by the user) the used method "getOppositeIncoming" should be revalidated
Computing priorities#
 code: NBEdgePriorityComputer::computeEdgePriorities(...);
 postcondition: Each edge has a priority at the node it starts and ends at. The priorities at the start and the end node may differ. An edge has either the priority 1 (higher prioritized) or 0 (lower prioritized or all equal).
 reason: We need this to compute the rightofway rules.
 how
All priorities are initialised to 0.
 state: orange
#5: Geometry computation#
#6: Computing rightofway and traffic light programs#
#7: Finishing inner edges (optional)#
OLD#
Steps #9#10: Computing certain signalling/rightofways#
Steps #11#17: Computing and setting lanetolane connections#
Steps #18#19: Computing geometries#
I am currently concerned with the edge shapes, because even a eworld or a TrafficModeler export stops here in debug mode. It is in NBEdge::computeEdgeShape. (Actually, as far as I understand this method, it is about the lane shapes of this edge.) In this algorithm, the following three cases must be distinguished:
The edge shape intersects with the node shape. Then, the shape is trimmed to the intersection point. This looks quite reasonable.  
The edge shape is outside the node shape. The the present algorithm extrapolates the last line of the edge shape and enlarges the edge to the new intersection point. This looks reasonable to me as well.  
The edge shape is completely within the node shape. What should happen here? (The red dots show, what netconvert does at present.) 
So I am wondering, what the right behavior of netconvert would be in the third case.
Steps #20#22: Computing rightofway rules#
Steps #23: Applying additional offset#
Table#
Action  affects  

nodes  edges  connections  
Adapting the input  x  x  x 
'Building node clusters ' 
x  x  x 
'Removing selfloops ' 
x  x  
'Joining similar edges ' 

'Finding isolated roads ' 

'Removing unwished edges ' 

'Removing empty nodes and geometry nodes ' or 'Removing empty nodes ' 

'Splitting geometry edges ' 

'Guessing and setting on/offramps ' 

Moving network to origin  
TLS position computation  
Assigning nodes to traffic lights  
Joining traffic light nodes  
Computing connections between lanes  
Computing turning directions  
Sorting nodes' edges  
Computing Approached Edges  
Computing Approaching Lanes  
Dividing of Lanes on Approached Lanes  
Appending Turnarounds (optional)  
Rechecking of lane endings  
Geometry computation  
Computing node shapes  
Computing edge shapes  
Computing rightofway and traffic light programs  
Computing tls logics  
Computing node logics  
Computing traffic light logics  
Finishing inner edges (optional) 