Modern dogs are a human creation.
Modern dog breeds were all developed by selectively breeding for various traits.
Terriers, for example, were bred to hunt vermin (rats, etc.), so they’re typically small (for going into holes), tough, and stubborn. Herding breeds were bred to select for herding behavior. Herding behavior ranges from the border collie’s “eye” (staring at the sheep to intimidate them) to the Australian Cattle Dog’s nipping at the heels of cows to move them. Sighthounds (greyhounds, Afghan hounds, whippets, etc.) were bred to course after game they could see. You et the basic picture.
Expression of the traits in a particular breed can vary quite a bit from one animal to another. I have 4 dogs (2 purebred Australian Shepherds and two Aussie mixes). One of my purebred Aussies has a LOT of herding instinct. When she sees a group of smaller animals* she wants to circle around them and keep them rounded up together. My other purebred Aussie had no interest in herding at all. The two Aussie mixes also have no herding instinct.
I believe a lot of the pit-bull breeds WERE bred for dog-fighting. As Nametag said, this normally means aggression toward other dogs.
Many dogs can be trained to be aggressive towards people. But for most well-socialized and well-bred dogs, aggression toward people is rare.
*Keep in mind that a running child looks a lot like a Small Running Prey Animal to many dogs. My dog Sasha certainly thought so the day she tried to round up a group of kids playing on the lawn.