In my experience if you're clean-installing an OS from USB (or DVD) it doesn't matter what OS(es) are on the HDD: they don't run. It only matters if the installer accesses the old OS for some reason. Perhaps your Win 11 installer looks for Win 10, doesn't find it, and just treats Win 7 as 'used disc space'.

If your machine is set to boot first from USB (boot sequence in BIOS) then on first-reboot the machine will do that unless installer overrides it to boot from HDD. I don't know how Windows installers do that - maybe under UEFI on new machines it can issue a 'reboot from HDD' command. Perhaps Win 11 relies on that feature, but your machine doesn't have it.

I've read that MS have said that if you have Win 11 on 'unsupported hardware' it won't get updates.