I think you can track pretty well with just the device ID alone, but you combine it with other identities like location information, IP address, cookies, login information and so on. Is that why Windows 11 recently made it impossible to bypass creating an MS account?
You estimate how many people use computers and how many people use computers you make. When you subtract the two, you get how many people use computers that your competitors make.
Not every “internal metric” is something you can measure directly.