Liftoff!

Tap on any community that you like (either the name or icon) and you'll see a fancy subscribe button at the top. And you can find communities via the search icon in the footer

Click the community name above the post, for example [[email protected]](/c/[email protected]). You should see a subscribe button there.

You can search for a community by going to the app's main screen and pressing on the magnifying glass icon. There, select your instance and enter a search query. From there, select Communities for a list of communities from your search result. If you want to discover communities, just enter a space in the search query and hit search, it should give you a list of all communities (in that instance I suppose).