Cookies are still the primary deterministic identifier in the desktop and mobile web world. Cookies can either be first-party cookies or third-party cookies.
Apple Safari has been the most restrictive browser and does not allow setting of 3rd party cookies by default on iPhones (though users have the option to alter this behavior from their Browser Settings), and have dramatically limited the lifespan of even first-party cookies with its ITP updates.
Cookie formats are typically non-standardized as most companies maintain their own cookie pools. They are also pseudonymous – that is, they can be tied to personally identifiable information (PII) but when viewed on their own, don’t tell the viewer anything beyond a string of letters and numbers.
