https://arstechnica.com/gadgets/2023/12/beeper-mini-on-android-claims-to-have-reverse-engineered-imessage-compatibility/

In the past week, I have sent an iMessage to one friend from a command-line Python app and to another from a Pixel 3 Android phone.

Sending an iMessage without an Apple device isn't entirely new, but this way of doing it is. I didn't hand over my Apple credentials or log in with my Apple ID on a Mac server on some far-away rack. I put my primary SIM card in the Pixel, I installed Beeper Mini, and it sent a text message to register my number with Apple. I never gave Beeper Mini my Apple ID.