Legacy BIOS mode, aka CSM (Compatibility Support Module) is the same thing. Name depends on UEFI firmware, most manufacturers use CSM.
All these are not necessary and he risks damaging his working Windows 11 installation. First make sure the Windows 11 disk has higher priority and boot the PC. If that fails, check again by pressing F11 (or F8 or whatever, check User Guide) to get the Boot menu and select Windows Boot Loader. If that fails, probably the drive letter is not C as it should, just use the appropriate tool in Strelec (see my earlier post) to fix the drive letters, so Windows 11 disk boots properly.