I have a question about your video:

The parallax (explained at 31:00) is only evident in scopes, not red dot sights, holographic sights, or iron sights?

Simple book definition: Parallax is a displacement in the apparent position of an object viewed along a line of sight.

An optical sight is constructed with a series of lenses. If the reticle and target appear in the same focal plane, a slight shift in your head/eye position will continue having the reticle/target image appearing in the same relationship. If the reticle and target are NOT in the same focal plane, this head/eye movement will give the appearance of the reticle moving in relation to the target even if the rifle/barrel is not moving.

With optical sights, the objective lens focuses the target image into the same plane as the reticle and needs to be checked for each distance shot. Checking for and making this focus adjustment will take out the parallax. The ocular lens focuses the reticle and is adjusted once for the shooter.

Red dot and holographic sights are theoretically parallax free. In practice, red dots may have some parallax inside 50 yards. Keep the dot/reticle centered in the tube and parallax is eliminated.

With iron sights, parallax error is eliminated by focusing on the front sight to keep it centered in the rear sight.