As far as official and app goes, there is m.lemmy.world. You can select Android UI or iOS UI in the settings. But there is no official app. And Jerboa is developed for Android by the same developers as Lemmy itself.
I don't know that it's actually considered to be the official Lemmy app, but Jerboa for Lemmy is developed by the same people who are the main developers of Lemmy. I'm currently using Sync for Lemmy, and have definitely found it to be the best Lemmy app that I have used.
The whole reason we all ended up on Lemmy is to have freedom from official apps. So far Lemmy has managed to cultivate developers that created several excellent apps.