How iMessage does it: https://support.apple.com/en-gb/guide/security/sec70e68c949/...

WhatsApp secret conversations don't carry either over multiple devices if you don't activate sync with google drive (which kind of defeats the purpose of encryption anyway).

that tells you a little something about how their encryption keys work :)

Yep, it's still terrible, and still refuses to load non-encrypted messages for you.

I just got onto Telegram this week and was pleasantly surprised to see completely native apps across iOS, iPadOS and a full featured native Mac OS X app.