this is currently... a complicated problem to solve.
unfortunately, with the current LemmyAPI implementation, there is a delay with communities between instances getting registered. so, let's say you're on lemmy.ml and you see one of those New Community posts from lemmy.world. well, those links, despite using the correct format to link to your local version of the community, typically results in a 404 page-- even in the web version. There's a workaround to get your home base instance to register it and then it will e searchable for everyone in your instance, but it doesn't always work, and programming it into the app is tricky and unreliable.
so what do?
even on the web version of lemmy, this is a tricky problem to solve, and we can only hope the hard-working lemmy devs resolve this issue in the upcoming update to the Lemmyapi at the end of the month. Until then, we're working on a community search and subscribe function (including favorites) that works similarly to the one on the website (and looks similar to the subreddit browser in Apollo in the left-swipe view). this should (hopefully) be in the next Mlem update!