Have a look at sample patches here - https://github.com/pingdynasty/OwlPatches , this should help you understand how things work.
Also, have a look at API docs
what the output function is called / how to send the processed signal to out L and R.
You should find example of that that in every C++ patch. You don't call anything, you just process audio buffer in processAudio callback.
the expression input
Expression input is treated the same way as other CV inputs, it's bound to PARAMETER_E.