m4ri0g dont' you offer a beta like google.
Google does not release the source code for the betas of their stock OS. The AOSP 15 source code has been released for a couple weeks but its not possible to fully port it to run on a device without the device support code. This only becomes available when Google do a stable release. They publish the AOSP code for devices the same day.
The GrapheneOS team have got a working port of 15 but various important things do not work without the device support code.