Where are the files (and code) behind a ArcGIS Enterprise Site?

[Arne_Gelfert]

I've created an ArcGIS Enterprise Site through Portal. This generates (at minimum) an item for your site (Site Application) and an item for your the Sites App Launcher (Web Mapping Application) in Portal.

I see some new entries under content on the Portal server, too.

But is there a way to look at any files, e.g. configuration, JS, css? If I create an app through Web AppBuilder, I typically get the app folder and am able to tweak the code. Where is all the code behind the Sites?