Ironically you can use it on android using an app called…wait for it….BlueBubbles. -Source: me using it right now.

It's not the blue bubble, it's that iMessage is like any other of the big message services and has it's own things like those messaging services. I have a group chat with another iPhone user, and an Android user. Just sending texts is whatever, but when you start using the messaging features like tapbacks to like a message or replying to a specific message, it gets real fucky real fast.

Although I am glad iMessage was created (largely as a fuck you to carrier fees) these days I wish that Apple would either make an Android app or the world (US mostly) could collectively move to a good messaging app/protocol (which includes e2e encryption and isn't fucking Meta.)