An easier way to think of it is to count 7 steps up from the tonic or drop back one step from the tonic. This changes the basic triad into a more complex chord and improves the sound. Seventh chords introduce dissonance and urgency of resolution.
It’s easy to confuse these. The Major 7th is the note a half step lower than the root. The Dominate 7th and the Minor 7th are two half steps lower than the root. Note: In the minor 7th , the 3rd is dropped ½ step.