I actually did this back in the late 90s! The editor was called "Scorpio"[1]. It was written for the classic MacOS in some version of C with objects, maybe Think C(?). I'm not 100% sure.

It's an amazing fun thing to do, but I probaby wouldn't wan't to do it again now. This thing didn't handle unicode (I had never heard of it), barely handled spell checking and didn't handle bi-directional input.

Text (1 byte per char) was stored in a big array on the heap. Styles were also an array (again on the heap) of fixed length structs. Font information, in the form of fixed-point width tables, was gathered from system calls and cached.

It did actually support inline pictures though, which was pretty challenging.

Writing an editor is a hugely fun project. Highly recommended.

[1] https://atpm.com/3.03/page11.shtml