Usually for these things there's downloads in the release section. For example, Jerboa's repository has an Android APK you can download directly from the releases page there. But you can also get it from F-Droid and Google Play.
You've guessed right, GitHub is for developers. If you don't want to be dealing with source code, you probably don't have any reason to care about GitHub.
Others have already covered your particular use case: you found the code for the web UI for lemmy, it's only useful to server admins. As a user you just go to the website, in this case lemmy.ml.