Your summary is right but is missing one key detail. In the period since iMessage was developed, every single direct messaging and group messaging app that got to scale has been x-platform across Android and iOS, and many of them have even been x-platform to web/browser clients and/or desktop apps across Windows, macOS, and Linux.

Namely: Telegram, Signal, LINE, WhatsApp, Discord, among others.

That is, all except one.

The one exception is, of course, Apple's iMessage.