Yohan Yukiya Seseㆍ사요한・謝雪矢Based on what I've noticed:
You do see the reply of the person you are following, let's call that Person A.
Depending on the software you are using (for example, Misskey and forks), you will also see the post Person A replied to.
Replies of external accounts to Person A will also appear, as long as those external accounts are already followed by someone else in your local instance. (I think this also depends on the software itself.)
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