Use TWRP or Fastboot to flash the .img file of your chosen GSI to the system partition.

There are very few device-specific custom ROMs for the Y9 Prime. Instead, users must use Generic System Images (GSI) Android Versions

What are the best custom roms available today? : r/androidroot

Huawei Y9 Prime Custom ROMs: The Ultimate Modern Guide The is a legendary device known for its notchless "Ultra FullView" display and pop-up camera. However, as official updates have slowed, many users are looking toward custom ROMs to breathe new life into their Kirin 710F-powered hardware.

Unlocking the bootloader is mandatory for flashing any custom recovery or ROM. Since Huawei no longer provides these codes for free, users typically rely on two main alternatives: Paid Services : Tools like HCU Client DC-Unlocker