In web to web attribution, providers can use identifiers stored in the cookies of the browser to identify users across multiple sites. In app to app attribution, providers use the advertising identifier (IDFA or GAID) to recognize users from app to app.
When matching a browser click to an app install, attribution companies will generate a common identifier called the device fingerprint. This device fingerprint can be made up of multiple device specific attributes including IP address, operating system, operating system version, and screen size.
See fingerprinting to learn about its deficiencies.