Please help me solve the problems of milk and dairy industry. Thank you.

I study food, and I can only solve your last two problems. Please forgive me.

There are all the methods you mentioned. Large dairy manufacturers will have their own milk factories, but this is not enough, so they will buy or cooperate with dairy farmers (most of them are at present). In fact, most of them are bought from dairy farmers. It is also common to exchange milk powder for milk. It is also possible to recycle expired milk and use it to produce yogurt, so basically, you have said it.

3. Pasteurization is actually low-temperature sterilization (including high-temperature short-time sterilization and ultra-high-temperature instantaneous sterilization). Sterilization at 55-65 for 30-60 minutes can effectively preserve the nutrition and flavor of milk (for milk, the protein content is very high, so this method is the most suitable), and it can kill most bacteria, which can only be regarded as disinfection, but not as complete sterilization, so, ah, there are residues.