Also not a lawyer or a US person, but from listening to American tech media, this has been an issue of some debate for a decade or more now.
The trick lies in their 5th amendment right against self-incrimination. Police cannot require you to give your PIN because that would violate 5th amendment rights. It has been ruled in some parts of America (but the ruling in other parts has been the opposite, IIRC) that you can be forced to give biometric unlocks. In my opinion this is kinda silly and inconsistent. It might be in line with the letter of the law, but it’s certainly not in keeping with its spirit.