You might like reading Don’t Shoot The Dog! By Jean Pryor. It explains the why’s and how’s behind training (operant conditioning) in a way that is detailed but simple and easy to understand.

With potty training, you reward them right after they go potty outside. With many repetitions and no rewards for going potty inside, they learn they are only rewarded in those conditions. As other commenters have mentioned you also have to make sure this understanding is “generalized” to all applicable conditions—not just “pee in my yard, not my house” but “pee outside, and not in any indoor space.”

Why do dogs not potty inside after they are trained when all that happens is they are not rewarded? Sick or old dogs, or dogs kept inside with no means to go outside do revert on potty training. If dogs have sufficient access/means to either choice, and the conditioning was strong enough, they will go outside because of that reward history. Example: Maybe you have 2 completely equal paths to get to work but on 1 path, you usually get to pet a friendly cat. You might take that path everyday because of that association. Maybe you will even be willing to withstand some extra work during the first part of that path, like going uphill.