I am not sure how to make it more simple than that for absolute beginners.
Not sure your "Amazing Grace" midi example will work well. BIAB will inactivate styles when you load a midi. So what you say is true, "...the midi will play on the melody track", but will leave you with an expensive midi player. BIAB is one tricky app.
The way I learned BIAB as a newbee was with the 1000's of BIAB-formatted songs which populated the web. Today, not so easy to find. So for those newbees purchasing now, merry xmas:
BIAB xmas Songs