About to be a buzzkill but I feel that some of you will benefit from this:
"How did people discover cow/goat/whatever milk? Like what were they doing?"
Human babies breast feed right? Like they have for a shít ton of time, like since babies have been a thing.
Well someone probably saw a calf (baby cow) feed from it's mother and made a connection like that, and tasted it, and liked it, and spread it around his village or whatever and tada, milk.