Rstanarm function into stan code

I am working on modifying an rstanarm model. I can expose the underlying Stan model, but it seems like I’m missing how the data is being translated from an input for rstanarm into an input for the underlying Stan model. Can anyone point me in the right direction? Thanks in advance!

Might be easier to use brms package which handles rstanarm formulas, but has also make_stancode and make_standata which make more easily readable Stan code and creates the appropriate data structure for you