How do people know who sent them bitcoin, ex. in case of a ransomware targetting multiple locations?

Question

I have a rough understanding about how bitcoin works and have read about how anonymous it is, or rather how not anonymous it can be in certain situations. But I still do not get how is it possible to use bitcoin for transactions where it is important for the recipient to know who sent which amount while staying "anonymous" and not communicating with the sender. In case of a pirate expecting to receive money from more than one victim, how do they know who paid? Same question for people selling illegal stuff.

Disclaimer: I am not a pirate, nor a drug dealer. These are the first examples that came to mind.

Answer 1

The answer is simply that in almost all cases, the receiver does not care who paid. They care about what item/service/bill was paid, not by whom. And to do that, receivers create a new receive address for every payment they expect.

In the case of an online shop, when an order is placed, they create an address, store the address in their database together with the order information, and when that address receives the expected payment, the order is processed. It doesn't matter who paid it, or even whether that was the same person as the one who placed the order; it only matters which order was paid.