There is no official App for Mastodon.
Instead there are many different Apps.
(Also here the similarities with email are helpful: there is no 'official email App', right?)
The most famous Apps for Android are:
- Tusky
- Fedilab
The most famous App fir iOS are:
- Amaroq
- Toot
Every App has is pros. and cons.
Some are free and some not.
Some are also on F-Droid, the open source 'store' fir Android.
Check out which one you find the best for your taste.