I'm not going watch the video to hear yet another person regurgitating theories about the film that have been out there since 1968. We're no smarter than audiences and critics were then, and interpretation of the monolith as aliens, a movie screen, a doorway, etc. etc. ad nauseam are as old as the film itself.
I'm glad Kubrick never chose to ossify a single interpretation of the film by saying what it meant or what he intended it to mean. In fact, from my reading—and I've read a lot about the film's production—he was adjusting and adapting throughout shooting and post-production. The monolith wasn't always a black rectangle. In fact, they apparently had a see-though perspex monolith built, then scrapped when the results weren't satisfactory. They spent time and money and tests to try to portray aliens—including using the man who played the unnamed Moonwatcher covered head to toe in dots and having him move against a static pattern of dots—then gave up very late in the game when they realized none of it was working.
One thing that struck me maybe my second time seeing the film as an adult is that there are really only three sequences, not four, a triptych, not a tetraptych. The Dawn of Man, Jupiter Mission, and Jupiter and Beyond the infinite. The Dawn of Man includes the australopithecines and everything though Floyd and the astronauts getting eardrum-blasted. The iconic 4-million year jump cut just moves from the first piece of technology to a much later piece of tech. We watch the australopithecines sleeping, eating, fighting, stagnant, and curious about this thing that shows up amongst them, same as Floyd those he encounters on his trip, whom we see sleeping, eating, fighting (in a Cold War manner), stagnant, and curious about this thing that shows up amongst them.
As has been speculated endlessly, HAL can be read as the ultimate expression of that bone tool, and Bowman has to kill it, let go of it, and be reborn in order to become the Nietzschean ubermensch we see at the final.
But that's just one of endless possible interpretations.