They both have the opportunity to upgrade it. The BIOS holds a copy of the microcode and this can be upgraded by upgrading the BIOS/EFI (bundled with BIOS upda

They both have the opportunity to upgrade it. The BIOS holds a copy of the microcode and this can be upgraded by upgrading the BIOS/EFI (bundled with BIOS updates). The same for the OS. On boot the BIOS loads it's update first, then the OS, provided each is newer than the currently loaded microcode. So the idea is that if you don't like this update, you need to prevent your BIOS from updating (or downgrade it), and then also configure Linux to ether not upload it's microcode, or load a specific one.