Beeper Mini is an Android app that prioritizes privacy and security. Transparent and open source, it encrypts all communication and ensures that messages cannot be seen by Beeper or Apple. The app connects directly to Apple servers, eliminating the need for a Mac server relay. Beeper Mini does not require an Apple ID and does not have access to user data. By reverse-engineering the iMessage protocol, the app has implemented end-to-end encryption natively within the Android app. Although Beeper Mini does not use the Matrix protocol, support for Matrix network and other chat networks is planned for the future.
https://blog.beeper.com/p/how-beeper-mini-works