Ok, as somebody that doesn't know NFHS rules really well anymore, but who still observes, etc., let me try to paraphrase my understanding of the FBZ, BBW, IBB, etc. This, of course, is with the caveat that we haven't seen the final 2021 rulebook and casebook language yet. Corrections/refinements to my paraphrasing are welcome. This is basically for my own use to understand the differences between Fed and NCAA.
The FBZ measures 3 yds vertically and 4 yds wide from the spot of the snap, and any part of their body puts them in the zone. In practice, given a balanced formation and normal splits, this means TE to TE.
Offensive AND Defensive linemen in the zone at the snap can immediately block each other below the waist. It doesn't matter in regards to blocking below the waist if the QB is in shotgun or not. The block can be directed forward or from the side. Immediate, as a practical matter, means the lineman directly ahead or directly adjacent to them. They still can't CLP or CHB.
The other rule that uses the FBZ is blocking in the back. Offensive lineman in the FBZ at the snap can legally block in the back (above the waist) any defensive players that were in the zone at the snap. The block has to occur in the FBZ and the FBZ disintegrates when the ball leaves the zone.