Difference between Query collection, Document from reference and Generate dynamic children

Hello beginner here,

I've been struggling to put these into my logic so it'd be really nice if someone explain these actions with simple terms and tell the differences or where to use them. Or maybe show some materials to check out since there isn't much on Youtube.

Also there is this User Reference (Users ref) tab under Authenticated User when you select Set from Variable, which again I can't seem to understand logically.

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