Finally, Mark was inside TWRP. He wiped the Data, Cache, and System partitions (a crucial step he almost forgot). He transferred the ArrowOS ZIP file and the GApps (Google Apps) package to the phone.
If you manage to unlock your bootloader, here is how a custom ROM compares to the stock EMUI 8.0/8.1: huawei y7 prime 2018 custom rom better