The Re in ReVanced is for "Reverse Engineered" Vanced. ReVanced is going to be identical to the original Vanced, but later on (especially since it's open source) more patches/"plugins" will be available.

At first is different because to avoid the same issue as Vanced, it will not ship you patched version of YouTube App but it will ship a patcher.

That mean you'll must have installed YouTube because it will be companion or patch (I'm not sure if it will patch the YouTube app or be a service next to YouTube to apply Vanced feature at runtime)

don't know for sure, but since Vanced wasn't open-source, ReVanced possibly decompiled it and extracted the patches?