You have to tap the community name in the overview of posts or the thread itself. Then there is a subscribe button at the top.

If you open the community (tap the link) then there is a big button saying join

Just for completeness: You can search with the "menu" icon in the bottom bar. Not all communities are federated yet, so you need to search for others on https://browse.feddit.de first and then search the exact full name of the community in your instance (didn't work in the Jerboa search for me)

By exact I mean e.g. !indiehackers@lemmy.world (including the !)