It does, in fact, relate to a repo like a repo relates to a file! Because every projection only contains related history and subtrees – much like if you were looking at a history of a file in a git repository
You can, if histories are related. They usually are - you add commits on top. Your history will be reverse-transformed in such a way so that its impact on the whole metarepo can be assessed. So if you modified a file in a library that ten other applications depend on, those applications would get this modification applied and would trigger a CI run to verify your change.