A universal ID is a unique user ID that allows AdTech companies to identify users across different websites and devices.
Universal IDs are created using IDs stored in cookies, typically first-party cookies, and device IDs from smartphones, tablets, and other devices.
Some universal IDs operate within one environment, such as web browsers, while others aim to identify users across different environments, such web browsers and mobile devices. For the latter, device graphs are used to match together the IDs generated in web browsers with the ones generated in other devices, e.g. mobile IDs in smartphones.