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.

Yes, Sharkey is a microblogging service and it can’t create headings, but it still displays them correctly. Among other things, this is better than with Mastodon.

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.

Yes, you can do this from almost anywhere in the Fediverse. As Mastodon does not recognise headings, the first paragraph becomes the heading. This means that it looks better on Lemmy if you put the community tag at the end of the post and start with a content-related introduction.

Let’s hope that there are also a few organisations that run Friendica instances so that the offer becomes a bit bigger. In the last two years, everyone has been totally fixated on Mastodon and many have opened a Mastodon server without realising that there were other options.