How do taxes work with Prolific?

As someone new to doing taxes, since Prolific pays in GBP and I live in the US, do I have to report my earnings in GBP or USD? I assume USD but I know tax policies can be weird sometimes.

In addition, I know many beermoney sites have a limit, usually $600, that you can earn before they send you a W-9. Is Prolific like this, or is it different because they’re a British site?