Am7 is an often used seventh chord. It has four notes - LA-DO-MI-SOL. There are a few common ways to play it in the 1st position.

Amaj7 is a major seventh chord so it has four notes - LA, DO#, MI, SOL#. This chord is built from either the first or the fourth step of a major scale. So it can be the tonic of A major or subdominant of E major.

Dm or D minor triad is an open string chord and consists of three different notes (re, fa, la or d, f, a). The bass is RE (D) and therefore you mustn't strum the other bass strings.

Em or e-minor is one of the easiest chords to play on the guitar. Although this is a triad, we play all 6 strings and most of them are open strings.