A A Minor 7 (Am7) chord contains the notes A, E, G, C. It is built from the minor 7th formula 1 - ♭3 - 5 - ♭7, with A as the root.
What notes do Am7 and Cmaj7 share?
They share C, E and G — three of their four notes. Am7 adds A beneath them and Cmaj7 adds B above, which is why the two chords blend so smoothly and sit next to each other constantly in jazz and lo-fi writing.