The mastodon apps work, but generally only offer the mastodon functions. Fedilab may offer a little more.

There is also Dhaaga: A "client for various social networking platforms and protocols.

🩵 Bluesky 💙 Mastodon 💚 Misskey 💛 Pleroma 🩷 Sharkey 🧡 Cherrypick 💜 Akkoma"

edit: sorry, forgot to read that you are looking for iOs

There is already a more up-to-date version of the second graphic (including mbin and loops):

https://media.kuketz.de/blog/empfehlungsecke/Fediverse.png

Yes, Lemmy accounts can only follow/subscribe to groups (besides Lemmy also mbin, piefed + also Peertube), but not individual accounts. A Lemmy account can only communicate with individual accounts if they write something in a community. That is one of the aims of mbin, to connect them: The structure of communities /magazines + a microblog feed.

Ok, with all pictures in replies it doesn’t work yet or only in the other direction: This Lemmy thread seen from Friendica + Mastodon

Friendica shows all pictures of Lemmy, both in posts and in replies. The other way round only in OP.