https://www.raspberrypi.org/forums/viewtopic.php?t=275370
This is a beta test of the Raspberry Pi OS (64 bit)
We still recommend the 32 bit operating system for all Pis at this time, although have decided it is now time to begin the move toward a 64bit OS. For the moment this is a 'beta' program, the OS is in heavy flux and its functionality is likely to change significantly over the next few months.
Note, the 64bit OS is only install-able on the Pi 3 and Pi 4 devices
Latest 64-bit image: https://downloads.raspberrypi.org/raspios_arm64/images/
https://www.raspberrypi.org/forums/viewtopic.php?t=275370
Latest 64-bit image: https://downloads.raspberrypi.org/raspios_arm64/images/