IANAL. This is my current understanding. If I'm wrong, feel free to correct.
Using it in a product counts as "public disclosure". If you've shipped it, that's a public disclosure for patent purposes.
But it's more complicated than that, because you can file for a patent on something within one year after the first public disclosure. That also means that you can file within one year after someone else publicly discloses something, if you lack morals and decency.
So if person A develops something and starts shipping it, and person B files more than one year after the first shipment, then the device is now "prior art" that invalidates the patent. Person A faces a legal fight, but they shouldn't be infringing, because the patent should be invalid.
Just to be clear, in the US you must be an inventor to file. You can't take someone else's invention, once publicly disclosed, and then patent that within the one year period. But if two people have same invention the first to file usually has precedence.
Of course in practice there are thousands and thousands of trash patents but technically those are the rules.
Using it in a product counts as "public disclosure". If you've shipped it, that's a public disclosure for patent purposes.
But it's more complicated than that, because you can file for a patent on something within one year after the first public disclosure. That also means that you can file within one year after someone else publicly discloses something, if you lack morals and decency.
So if person A develops something and starts shipping it, and person B files more than one year after the first shipment, then the device is now "prior art" that invalidates the patent. Person A faces a legal fight, but they shouldn't be infringing, because the patent should be invalid.