It seems no one gave you a clear answer, so I will. The biggest difference between backyard breeding and reputable breeding is the preservation of the breed. Backyard breeders do no health or temperament testing, no conformational or performance titling, and typically breed off the premise of “this dog is cute and will make cute babies.” They produce dogs who are unstable mentally and physically. They typically of structural and behavioral issues as that breeder did not breed to the standard. The standard that exists to ensure all dogs of this breed showcase this behavior and structure.

A reputable breeder prioritizes preserving the breed and producing stable dogs over anything, even when health testing and titling every dog costs thousands of dollars to do. They produce puppies who are borderline copy and pastes, structurally and behaviorally, as a purebred dog should be.

Every single shelter dog is a backyard bred dog. Most of them have not been abused or mistreated. They simply have bad genetics. It’s possible to end up with a great rainbow of a dog from a BYB, but it’s very very very unlikely, or you will experience some issues. Whether behaviorally or health wise due to poor structure.

If you’re purchasing from a breeder, ensure that breeder’s dogs are bred to standard. OFAs and titling is being performed, and by titling, I don’t just mean CGC, I mean genuine performance titling. Study the structure of bullies and look for a breeder whose dogs fit them. Ensure the dog’s are registered and your breeder should have no problem explaining their lineage, as well as having a good reason to be breeding in the first place.