Flashing a stock is different from a Samsung or Xiaomi. Huawei uses a proprietary flashing protocol. You have three primary methods.
Because the Mate 20 Pro launched with Android 9, it fully supports Google's Project Treble. There are very few device-specific custom ROMs (like LineageOS) built natively for the Mate 20 Pro due to the closed-source nature of the Kirin processor. Instead, users flash .Popular GSIs include: huawei mate 20 pro rom
If you're determined to install a custom ROM on your Huawei Mate 20 Pro and have successfully unlocked the bootloader, the following steps provide a general framework. Remember, the exact commands and files will vary depending on the ROM you choose. Flashing a stock is different from a Samsung or Xiaomi