Until someone on your instance subscribes to a community on another instance, yours won’t have any info about it. So it won’t come up in search results.
You can run a search for a specific community if you know and enter it’s exact name and instance. You might have to do the search twice. Once to tell your instance to look for it. A second time to see if it found it.
The place to find those specific community names and instances, is here
I’ve always been confused by people who insist on the All feed then come here complaining “No, not like that!”