The G sung over the F chord in the second bar makes for an added ninth harmony, while the same note over A minor in bars three and four gives that chord a minor seventh quality. In both cases, it's the tonic pitch accounting for the harmonic extension, and this centering around G anchors the song in its key while the chords (which are really more native to C major) are stewing about. The second part of the phrase is sung in harmony and the last two notes are on D, which is also not a part of that A minor chord but is, of course, the fifth of the "home harmony."
It's impossible to say what the contrasting section of this song is; its different set of chords give the impression of a bridge, but the part is heard with different words after both of the verses. (There is no chorus.) The more functional G major chord vocabulary here provides a sense of relief from the strange harmonic goulash of the verse, but the melody remains insistent with its extensions, emphasizing D as a major ninth over the C chord in measure three and as a minor seventh over the E minor chord in measure four.
