What is required for User ID to track users across different devices?
- A new Analytics account for reporting
- Google Tag Manager
- Sign-in that generates and sets unique IDs
- All of the above
The User ID enables the association of one or more sessions (and any activity within those sessions) with a unique and persistent ID that you send to Analytics. To implement the User ID, you must be able to generate your own unique IDs, consistently assign IDs to users, and include these IDs wherever you send data to Analytics. For example, you could send the unique IDs generated by your own authentication system to Analytics as values for the User ID. Any engagement, like link clicks and page or screen navigation, that happen while a unique ID is assigned can be sent and connected in Analytics via the User ID.
Read more here: https://support.google.com/analytics/answer/3123662