To my knowledge five #Fediverse platforms boast official #iPhone, #iPad & #Android apps:
#WordPress: https://wordpress.org/mobile/
#Flipboard: https://about.flipboard.com/download-flipboard/
#Mastodon: https://joinmastodon.org/apps
#MicroBlog: https://help.micro.blog/c/apps/8
#Minds: https://www.minds.com/mobile
Official #Threads iPhone/Android apps: https://about.instagram.com/threads
#ActivityPub platforms with just official iPhone/iPad apps:
I can not forget about #Funkwhale who officially is #Android only: https://www.funkwhale.audio/apps/
Yes, I am aware of official beta apps existing (#Pixelfed), as well as numerous third party apps, but as #Twitter (before #ElonMusk rebranded it as #X) discovered years ago, it is very difficult to become mainstream without an official app.
There might be other #Fediverse platforms with official apps, but these are the ones I am aware of that support #ActivityPub.