Processors
Here is an overview of onshape-to-robot
pipeline:

(1): The assembly is retrieved from Onshape, to produce an intermediate representation of the robot. See the robot.py from the source code.
(2): Some operations can be applied on this representation, those are the processors, some of them are listed below.
(3): The robot is exported to the desired format (URDF, MuJoCo, etc.) using an exporter.
Note
If you want to tweak your robot in the process, do not hesitate to have a look at the export.py script, which is the entry point of the onshape-to-robot
command, and summarize the above-listed steps.