Major 7th chords, and 7th chords are two different chords. A Major 7th chords is made up of the 1st note, 3rd note, 5th note and seventh note of a scale, so the Fmaj7 is made up of F, A, C, and E. A 7th chord is actually what’s know as a dominant seventh, and made up of the same notes except a flat 7th note, F7 would be F, A, C, Eb. Inversions are chords that use the same notes, but the order you play them in is different. The Fmaj7 in Raindrops keep falling on my head is played like this:
but Fmaj7 can also be played like this:
They all use the same notes, so they should all sound the same, but the order the notes are played and what note is the lowest in the chord are different. Aldrine covers inversions and extensions (7th, maj7 and more) in week 6.