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 ...
Harmonic minor keys are used when writing chords ... relative minor. So G major, for example has a key signature of one sharp, but this is also the key signature for E minor.