there are two different things going on: 1. In this category we can upwards clip which we do on 1:02 on top of the radiator, that skips a trigger that would start u being vulnerable. (At the start of the game you are invincible until this point) 2. at 6:02 we do a save delete to spawn the airboat, that would get rid of the invincibility but we also immediately do another trick where we exit the airboat into the level change trigger, making a part of our hitbox stay in the airboat (easy explaination) That lets us do these clippings with objects through walls for the rest of the run and also has the effect that bullets cant hit you anymore. However from this point onwards we can take damage from explosions which is very useful for grenadeboosting and stuff

Yes that part is commonly accepted that it's being one of the most satisfying bhop sections in the game

Why do you think there are categories for steam and non steam version of half-life? Silly. Using game bugs is a thing because it's often more fun, interesting and of course faster. If you want a glitchless run, watch a glitchless category. Though depending on the game, people might not want to run it cause it's too boring, so you might just have to content yourself with let's plays at 2x speed.