You can search by:
- Subscribed = only communities you've subscribed to
- Local = only the instance you're a part of
- All = all instances that have been federated with your instance, including your own.
You can also filter by specific communities, creators, or by keyword(s).
If you want to add a community from another instance that you can't find in search, you can either enter:
- !communityName@instance.name Or
- Paste the full url into your instance's search bar. It may take some time to find, subscribe, and sync to it though.
You can search for more communities at browse.feddit.de. Also, it's easier to just do this from a browser rather than the apps available.
As for apps- Jerboa for Android and Mlem for iOS.