[2019] What? How? Can someone explain ducking mechanics in 1st and 3rd person?

This is a thing in singleplayer only, your 1p hitboxes are different from 3p hitboxes. Again, this is singleplayer/local play only. Online play only the 3rd person hitbox counts, even if you're in 1p.

The explanation is very simple. In singleplayer, we don't run a shadow copy of hitboxes for the local player who is in 1st person (and using 1st person animations). Online, the server calls the shots, and the server uses 3p hitboxes and animations only. EDIT: the reason we don't do this in singleplayer is because introducing a mirrored set of hitboxes would add considerable complexity to the game's overall code and logic for little benefit. The fringe cases where this is problematic are largely irrelevant for local play.