This! The breeder basically keeps two parental lines that they use to make the seeds. Usually they need to do the crossing pollination by hand somehow and make sure that no foreign pollen fertilize the females. There is a great and accessible book about breeding by Carol Deppe if you’re interested in breeding plants in generally https://www.chelseagreen.com/product/breed-your-own-vegetable-varieties/

I’ve also always been wondering about this and other questions before reading that book.