routing parts can be modelled like any other part using extrudes & cuts. The only additional requirements being the setting up of route points and connection points. Once done the part can be saved to the library.
See the menu item Routing
drop down Routing tools
Easist way to add CP and RP points as well as the rest is by using the wizard
sub selection Routing Component Wizard
Actually I've found that the easiest way to make a routing fitting part is to find a similar one and modify it…
True it can be helpful to start from a similiar one but be carefull of what fitting you start with.
I have run in many a problem starting from something that came standard with SW because of issues inherent to how it is made.
This due to errors in it, convoluted way of design, designed mainly for estetics, etc.
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