I am confused at which stage this is happening.

Is this after the bootloader, after initramfs but now systemd-cryptsetup is loaded and unlock the first disk?

AFAIK when I do my first disk unlock, at that point does systemd units get loaded including systemd-mounting.

Those mounts can already already mount/unlock encrypted secondary disks, based on the keyfiles stored on the now decrypted disk. So what exactly in this case is the advantage of any of this?

EDIT: Also, is there any discussions over ftpm support? Last I checked TPM2 was ok, but ftpm (which most intel/AMD now using) are a bit flaky in regards to support.