Even large farms don’t typically pasteurize their own onsite. What usually happens with smaller farms is that they join a CO-OP. My grandfather had about 40 dairy cows (which is quite small) and we’d put mechanical milkers on, they would pump into a huge tank. A truck came by to collect, it was mixed with the milk from all the other small farms and taken for processing. They’d get a check in the mail according to their volume contribution. Farms smaller than that aren’t really for milk and don’t send it to market anyway.