Major and minor chords are made up of three notes - the 1st, 3rd and 5th notes of the scale.
The 7th chord adds the seventh note of the major or minor scale to the existing triad. So it will contain the 1st, 3rd, 5th, and 7th notes of the relevant scale.
We can therefore have a major 7th, a minor 7th, and one other, a dominant 7th
The major 7th contains the 1st, 3rd, 5th and 7th notes of the major scale.
The minor 7th contains the 1st, 3rd, 5th and 7th notes of the natural minor scale (or we could say it contains the 1st, b3rd, 5th, and b7th notes of the major scale).
The dominant 7th contains the 1st, 3rd, 5th and b7th notes of the major scale.