Building a 7 chord is just like building the triad, you just add one more note on top. Root, 3rd, 5th, and 7th. In the key of C it looks like C, E, G, B.
The minor 7 chord: in the key of C major – D, F, A, C = the 2m7
5 chord = G, B, D, F. This is also known as a dominant 7 chord. Sometimes when you add the 7th note it can be further away than at other times e.g. the major 1 7th chord is 11 half steps. The 7th of the 5 major chord is 10 half steps. A dominant 7th chord is a shorter chord than a major 7th.
In the key of C major we cant have a G major 7 chord because that would be adding an F sharp and that’s outside of the key of C major, so it must be a dominant 7 chord (G-F).
The 1 and 4 chords will always be major 7 chords. The 5 chord will always be a dominant chord. Minor 7th chords will always be minor 7th chords.