Thunder is an open Lemmy app for iOS and Android that is currently under development. You won’t find it in any app store, so it’s not as well known as Jerboa. Still, it’s an excellent app.
The client is written in Flutter/Dart. If you have any development skills, lend a hand!
I’d really like to see this in TestFlight, but it’s cool to learn that AltStore is a thing. Gonna check them both out when I get a chance.