What happens to themes when you install an app?

[KnutA]

I'm trying to understand how installed 3rd party apps affect the different themes.

Is it so that when I install an app, code is normally injected into (only) the published theme? And not into the previous versions (those that are not live)?

If so, does this mean that it may be a good practice when one installs an app, to first duplicate the theme, so one may go back to the backup if one decides not to use the app (uninstall it)? Because code is only injected to the live theme? (To avoid code lying around from an uninstalled app, perhaps affecting the website).

But if so, why can I preview previous themes, and the apps will be there, even if code isn't injected into them?

I'm sure there are recourses on this, I just struggled finding them.

Thanks!

-Knut