RPi3 hardware is 64-bit capable but media drivers (inherited from RPi1/2) are optimised for arm and on the advice of RPi devs we continue to ship RPi3 as an arm image. RPi4/5 run different drivers that are written and optimised for aarch64 or where the small performance difference doesn't hurt.
If you disable auto update mode (which only auto-updates minor updates) in the GUI and then manually choose to update to LE12.2, the file downloaded and used for the update is the .tar file. If you download from the website the .img.gz file can be used.