iMessage does end-to-end encryption by default, but only between iMessage clients (blue bubbles). I believe (someone correct if I’m wrong) that android’s basic messaging app does SMS by default, which does not support encryption. So iMessage can communicate with android phones, but only by falling back to basic unencrypted messaging, which it displays in a green bubble.

There are other proprietary iMessage features too, like animated messages, that don’t work for android at all. And I suppose to be fair to Apple users, it is kind of annoying having an android in a group chat because tagging a message (thumbs up or whatever) doesn’t actually tag it, it just sends a message that says says “so-and-so liked a message”. Not worth bullying someone over, but I get the irritation.