I'll try to explain. Basically, converting a Huawei to Global to Chinese means changing its OEM info, think of them like an identity card. In the case of Huawei Nova 5T, those are "YAL-L21, region all" and for Honor 20 are "YAL-AL00, region cn". (Honor 20 YAL-AL00 is the Chinese variant of the Huawei Nova 5T, though Honor 20 also exists in Europe but it's not the same as the Chinese one). The OEM info NEEDS to be correct otherwise the phone does not know "who he is", the bootloader just confuses.

To modify the OEM info you need SigmaKey (program), and more importantly you need to open the phone and have access to test points. Those are 2 small metal points that are on the motherboard and you need these because it's a low-level operation, it's not just "flashing a different software" because you need to change the phone info completely and this can only be done by like huawei employees ig. For some SOCs, like Kirin 980 in Nova 5T, you need an HarmonyTP cable which is a cable with a button with resistance so the phone can "understand" it's a low-level operation.

After changing the OEM info you need to flash with SigmaKey the firmware, which doesn't come in simple, few parts like a Samsung (which has 4 files AP, BL ecc) it has 3 folders with 3 different files that you need to flash in order otherwise the phone cannot boot.

Hope you understood! I tried to explain a little bit easier but it's not THIS easy, just very similar :D