Unfortunately you can't downgrade to Windows 10 on a PC that came with Windows 11 in the first place. I also think that even when one upgrades a Windows 10 machine to Windows 11 it's not a given that a downgrade it's always possible (the upgrade process actually creates a new Windows digital license for the same PC).

I also do not know whether the OLD Windows 10 key that was valid before the upgrade to Windows 11 (should one happen to have written it down somewhere beforehand), would still work on the same PC. It probably should but I am not 100% certain as Microsoft could easily render that key invalid. Kind of a mess, no?