The minor seventh chord is created by adding a flattened seventh note to the minor chord.
i.e., Minor chord formula: 1 b3 5
Minor seventh formula: 1 b3 5 b7
e.g., The Gm7 is based upon the notes of the G major scale.
G Major Scale
Here is a common movable shape for the m7 chord shown here at the third fret i.e., Gm7. This is a root 6 shape.
Gm7 (Root 6)
The 5th string is deadened by the second finger. The first string is deadened by the third finger.