Android is also Linux though

Syncthing is open source, so I’d personally prefer it. https://github.com/pixelspark/sushitrain is an iOS client written in native Swift.

Briar is Android only. Bitchat is an iOS app (may have an Android port in the future though, I think).

That’s only one use of secure boot. It’s also supposed to prevent UEFI level rootkits, which is a much more important feature for most people.