Firmware only zips for Poco F1 to change firmware version without touching system or vendor.
Firmware zips here don't have system or vendor and don't touch them while flashing, so you can use this on any ROM, stock or custom to change firmware version.
Specially useful when you are facing touch issues or signal issues due to firmware. Try multiple versions and keep which one suits your phone the best.
Instructions: Flash in TWRP (or alike) recovery. Just reboot in recovery, flash and reboot to System.
You can upgrade/downgrade firmware without breaking anything (though there is no guarantee and to be uses as is) regardless of existing vendor or ROM version.