I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “!communityname@instance.domain”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
Am I right in thinking that once you’ve searched (or subscribed?) to a community on another instance it’ll show up in the ‘all’ feed for everyone else, too? It’s a good way to help your wider instance discover other communities!
Yes. Searching will have it appear but subscribing means comments and votes will start syncing over.