Mairi's Wedding

I don't understand AA or AB
In fact I don't understand music.
How you get 3 skip change steps out of 4/4 time

In Maiei's wedding
is the Chorus A
and Verse B

If so how would you play Mairi's wedding one time thru??

My object is to tell an accordionist, who has never played
Scottish Music for a Dance, who to play it

Thanks
John.