I'm not quite sure where this video is supposed to fit in. We already have amazing resources for beginners in Marxist's Precepts videos and Sigma's Momentum in TF2 Essay.
As for the video critique:
You waste 2 minutes of the video with some sort of advertisement for 6s, which is out of place for what your focus should be on: A Beginner's Guide.
Your class guides are either wrong, misinformation, or lacking in core concepts.
Your heal order is completely wrong.
Your communication section has a good premise, too many people overlook this aspect of TF2, and we really don't have a great video on communcation that current exists. The Master of Classes section with pyyour is a nice resource but is lengthy for an intro guide.
I dislike claiming that only certain classes can maincall; figuring out your maincaller should be a team thing not a you are this class you must do it thing. Calling focus is something that everyone (that can do damage) should do.
You gloss over the non-maincall comms to quickly, an example of each type of comm as well as describing why they are useful would be much better. The biggest thing I've noticed with new teams is that they fill their mumble with useless information.
I like the advantages talk, but you miss the other 4 important ones: spawn, class, positional, and ammo.
I like your discussion of using an uber, but killing the enemy medic is not the goal of every uber. Getting two picks, capping last point, staying alive, etc. also can be viable end goals of an uber. See marxist's uber videos for more information.
Health advantage primarily occurs at the end of fights or at chokepoints, however what makes health ad interesting is that even if you are down players/uber you can still win a fight. That is the key concept of health advantage.
Focusing should be done in any teamfight not just ones where health is a factor.
The core of competitive tf2 is capping points, not killing players.