Where the major seventh differs, is that it is only one semitone below the keynote, so in a chord of C major 7, the ‘major’ refers to the actual note and not the major/minor tonality of the chord.
Extended chords add other notes from the scale to the existing root-3rd-5th triad (for example, C-E-G), so if we add a 7th, we’d call this a major 7th chord (C-E-G-B). We can extend up to a 13th ...