Their “how it works” blog article is worth a read - they’re using a blackbox reverse engineering of the protocol (called PyPush) and re-implementing it natively in an android app, so there are no man-in-the-middle servers. It's pretty bonkers given how difficult Apple's spec-less tech can be to work with.
I’ve been using Beeper for almost two years now on desktop to view my iMessages, discord, and slack all in one app, and Apple hasn’t said or done shit about it yet