Thank you all, I went through the source code and the installer log.
All it does is flash boot.img and vbmeta.img to the current slot and does not touch the other. the recovery image does not work so I fastboot boot a working recovery and continue from adb shell.
Then it downloads the xz.tar files with their keys and push them to the device /cache/recovery once it's in recovery mode, then it adds recovery.fstab to /etc/
The downloaded files come with ubuntu_command file that is executed in recovery using system-image-upgrader script.
the script formats system_b partition and load keyrings then extracts the xz.tar files in cache, and strangely tries to move cache/data/* to /data when the first one doesn't exist, because the extraction results in system folder not data. (I guess I will change the path)
Anyway now that I'm sure the main slotA does not get touched I will keep testing until I come up with something that works. (I already got it to the boot splash but it keep loading and does not go past it)