Question about breed standards.

Question for everyone. I did some digging and found that english mastiffs are not a natural breed. They were purposefully bred by humans to have their characteristics and size. But my question is, how do breed standards actually exist or come from in the first place. How do you breed animals for specific traits if they don’t already exist in nature. To me that would be impossible because english mastiffs are a giant example of how humans can purposefully breed animals to obtain a breed standard but if breed standards came naturally then why would english mastiffs need to be bred at all? I get that with enough breeding you can create a breed standard, but the breed standard has to be based off of something in the natural world or else how the hell did english mastiffs get this big in the first place. So breed standards must come from nature right?