Bro, I know. Google came out with their system API like 3 years ago, but has been completely silent about their user API. Until they make a user API, only manufacturers can make an app that uses it.
I don't remember the full list as I've never used any or had any interest in them, but it was only OEM ones like Samsung's. No user ones, which could be solved by Google opening up the API.
The only one I can think of is Samsung Messages. I use it rather than Google Messages because I like the way that it handles custom notification tones for individual contacts.