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kotlinlang

actually, I see where the fragment is attached, it’s in content_main.xml

This is my point of view, just an observation, and it isn't founded in any android theory:

1. MainActivity is the "base" in your app, you can, if you want, name it otherwise. You may fire up other activities by using intents from your activity or its fragments.
2. Fragments (is at is says) fragments of your activity, they may load, unload, by your behalf. It may be killed while resting (not in foreground) if the system needs resources.
3. NavGraph is a handler for fragments in your app. It gives you an intutive way of making sequences or logically shifts of fragments. Your activity holds the NavGraph system. So I see it as a convenient way of handling many separate fragments in your app.

This is a nice way of putting it, thanks!