a tub drain has an over flow and a waste drain that tye together at the tub then it is usually connected with a compession joint to an 1 1/2" trap that goes to a san-tee that should not be more than 5' away.
No one knows for sure. There are theories, but absolutely none of them can be proved, and I doubt we will ever find out. The universe is expanding firther into what was here before the big bang....