Interval number; 1, 2, 3, 4, 5, 6, 7, 8 new octave
C scale...............C, D, E, F, G, A, B, C
So the E is the 3rd interval (we also use the word degree) in the C Major scale. The G is the 5th interval and the B is the 7th interval.
We build the chords of a scale by stacking 3rds. What that is saying is we use every other note in the scale to form the chords of that scale.
A major chord will use the 1, 3 & 5 intervals of the scale, thus the C Major chord is made of the C, E and G notes. If we take this stacking on out one more 3rd and gets the B note into the equation we have made the Cmaj7 chord. In my World I would say the 1, 3, 5 & 7 intervals were used to produce the Cmaj7 chord.
A minor chord will use the 1, b3 & 5 intervals of the major scale
, thus the Cm chord is made of the C, Eb & G notes.
A diminished chord will use the 1, b3, b5 intervals of the major scale
The Major pentatonic scale is made of the 1, 2, 3, 5 and 6 intervals of the major scale
. So the C major pentatonic scale will have the C, D, E, G & A notes.
If we sharp the 4th interval of the Major scale
we have produced the Lydian mode of the major scale. C Lydian has these notes C, D, E, F#, G, A, B. And if I was speaking in intervals I'd say the Lydian mode contains the 1, 2, 3, #4, 5, 6, 7 notes of the major scale