I understand that I will be shown the number of people subscribed on my server when logged in.
I don't understand, and I think that would actually be some kind of bug. If a specific server shows different numbers logged-in vs anonymous.
I still don’t understand why I cannot see posts made before I subscribed. I can see them on other communities and comment. Is that a decision made by each community?
Are you mixing up the words "community" with "instance"?
Instance means server. Lemmy.ml vs. Lemmy.world
Community means topic, such as !historymemes@lemmy.ml is a community
A Community has a "home instance" (server) that is where the community first originated and was created. Typically the home of a community is where the full copy of all comments, posts, votes are kept. But I suppose it is possible for even them to purge content to save storage or improve performance (or some other reason).
A Community can be subscribed to by remote instances. This is where there is a lot of end-user and new admin (new isntance) confusion. It is not automatic to get all content from all other instances. The very first Community subscriber on an Instance starts the flow of content from the "home instance" of a specific Community to this specific now-subscribed "remote Instance".
ALSO NOTE: there are bugs in Lemmy "replication" logic. Such as delete of a comment not going to all Instances for a community. As of version 0.18.2, the actual side-by-side comparison of two instances for the same Community can show such inconsistencies. I have personally run such comparisons with !lemmy_helper@lemmy.ml code. There isn't currently a mechanism I'm aware of to fix these issues.
Back to what you said:
I still don’t understand why I cannot see posts made before I subscribed.
If your remote instance has never had a subscriber to that specific Community, flow of data begins at the time of your subscribe. It does not do what is called "back-fill" of votes, comments, posts that may go back 4 years of Lemmy servers being online.
Does that sound like your situation?