Now, with some examination, I have some info to dump about the first hit of NAir. I am not an expert dataminer or anything, and I don't even main Ganondorf; I simply discovered this tidbit after fidding with his new NAir. Here is the guideline on hitboxes:
At the VERY end of his first hit of NAir, there is a hitbox that will slide the opponent forward and eventually launch at higher percents. The second hitbox will bob them a tiny bit forward, regardless of percent; this hitbox is Ganondorf's entire leg. Both of them deal 7%. There's also random 5% hitboxes that are irrelevant, as they do the same thing as the hitboxes mentioned above.
Followups after a fastfall hit 1 NAir:
Grab: if you hit the second hitbox, you can Grab > DThrow > NAir, dealing 33% (or 31% if you landed the 5% hitbox) at low percents.
Flame Choke: If you hit the second hitbox, you can Flame Choke > Tech-Chase.
FTilt: If you hit the second hitbox, You can FTilt which closes stocks at the edge of the stage at high percents.
DTilt: If you hit the second hitbox, DTilt will also end stocks at higher percents; useful if you aren't at the edge of the stage.
Wizard's Foot: If you hit the first hitbox at low percents OR hit the edge of the second hitbox at any percent, Wizard's Foot will end stocks at higher percents
Jab: Hitting the second hitbox allows for a Jab, a 14% string and a nice get-off-me attack.
FSmash/USmash/DSmash: Hitting the second hitbox and hitting off a read Smash attack will end stocks and mid-high percents.
Dark Dive: Hitting the second hitbox allows for a followup into USpecial, which will end stocks at the edge of the stage at higher percents.
Did Ganondorf's dSmash always do 21%? I remember it doing 19%.
It always could do 21%, because there are two hitboxes that do 15% and 13% on the second hit, and do 21% or 19% in total, respectively.