they precisely control every parameters for the end to end of bread-making process to make sure each batch of bread have that kind of consistency.

then small scale test lo. when the bread they make are that consistent in terms of quality, through small scale lab test can see what's the minimum and maximum date the bread is most likely to spoil.

then set the expiry date before the minimum perishing date. gaodim.

Fresh milk is ~1 month from factory.
Minus delivery/warehouse, usually when reach the shelves is about 2-3 weeks left.

UHT milk is several months, up to 6-9 months.

Soya milk without preservatives, can lasts about 3-5 days, max maybe 7.
Anything longer than that, confirm got at least some preservatives.
Easy, just buy some soya beans, grind it, boil and make soya milk. Then you let it cool down and immediate store in fridge. See how long can last.

Science. They dont pluck the numbers from thin air..

If you notice most food products they don't write "Expiry Date" but "Best Before". That is just the shelf date as a guideline for store owners. Most of the time even if after the best before date, the product is still safe for consumption.

There's just too many factors that contribute to how long the food can last, the safest way is to state a date much earlier than what the manufacturers expect the food will actually spoil.