Today’s video is just a fun move where you use a 6 chord (in this case, A6) as the I chord, then D7 and E9 for the IV and V chord.
By doing that, you maintain the same top note the whole time (the F#) which just gives a fun sound and makes the chords really flow into one another.
That flow from one chord to another is called voice leading, and it’s the practice of keeping the movement (in actual notes, not necessarily fingers!) to a minimum.
Jazz players are often really good at this, but most blues players could stand to think this way too, it sounds really great.
Enjoy and use it!