Help with sewage backup at home. how do I find the most downline drainage point?

It backs up in your tubs because they're on the floor which is the lowest point not because they're at the end of the line. Go outside and look for a cleanout close to an outside wall. It will be a plug with a square tang on top that can be unscrewed with a pipe wrench. Try running your snake in there first one direction and if nothing then the other direction.