When you click on the ‘All’ tab on your home page, you must have noticed communities from other instances gets shown. The total number of communities on the fediverse is much higher than only the ones shown however, and to make a specific community visible, any 1 person in the instance needs to copy its link and search it from here and subscribe.
Once someone does that, the local db fetches that information which will make it visible for everyone in the server.
So if you find an interesting community outside of this server, make sure to search about it here so its posts gets shown here in the future.
I know this might be confusing, so feel free to ask questions about this or something else about Lemmy in the comments or in a seperate post. Cheers!
Just a heads up, since I was having some trouble at first:
1 - Go to communities > search 2 - If you search for a given community and it doesn’t shows up, it looks like it is not indexed yet 3 - Search for the community in this format !community@instance (Like !foss@beehaw.org) 4 - After that, I had to go back to step 1 and simply searching for ‘foss’ returned what I wanted. But looks like the page needed to be updated.
Also, this tool might interest you guys. It is a universal community search in Lemmy instances:
https://browse.feddit.de/