Those steps aren't mentioned since they are part of the process of installing TWRP and can be found in their respective guides. BTW, never done anything about fstab.qc, only the vbmeta step.
Well, it's quite easy, the name should always contain "arm64", and "ab" or "a/b", if they are separated by a character or join it's a thing for every developer.
In the example you provided the string you gotta check is a64ab, which is for a64 processors which ab partitions. It won't boot on X2 pro, a64 it's not arm64.