The client code will be found in the page code and public files. Server side will be in the backend. You can start out with Velo: Where Do I Put My Code? for an explanation. The Wix Velo backend is based on nodeJS, for which you can find a ton of material by doing an Internet search.
I would suggest reviewing the Velo Basics . and working your way through the rest of the documentation.
Don’t worry - it’s really not all that complicated.