The Major Third.
The Major third can be found by combining the first and third degrees of a major scale.
You can create this harmony over any note by simply counting up 3 degrees of a major scale.
In the video, I began on C and counted up to the 3rd degree of the C major scale. If I had chosen F. I would need to use the 3rd note of the F major scale. Which would give me F and A.
Or if it were G. We would wind up with G and B.