There's 3 very different scenarios. I'll use server1 and comm@server2 to illustrate
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Nobody on server1 is subscribed to comm@server2. Until and unless someone searches/subscribes in the community page for the full URL (https://server2/c/comm), nothing from that community is pulled in at all, and going to the link results in a 404 community not found error.
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Somebody on server1 searched for comm@server2, but did not subscribe. A few of the top posts are pulled in, but nothing too heavy. The comments & votes are not guaranteed to be accurate.
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Somebody on server1 subscribes to comm@server2. Every post, comment and vote from that point forward is pulled in, the user counts will normalize, and generally after a few hours it'll look the same (except for some missing historical posts from before the subscribe).
Does that answer your question? Happy to clarify further if needed.