The follower on the mag contacts the bolt catch when it’s empty and pushes it upward. The bolt then contacts the bolt catch. That’s why if you have no mag in the rifle and pull the charging Ing handle and release, it won’t lock back. You can manually lock it back by manipulating the bolt catch by hand. Put an empty mag in and perform the same pull and release, the bolt should lock back as the follower has pushed up the bolt catch. This is another area of the 308 that caused some headaches for a while and if anyone is having bolt catch issues there a lot of posts on the topic that can be searched.
I had to stop watching the video halfway through. That guy drug a 30 second concept out for 11.5 minutes.