GA4 Effective user ID is a unique ID associated with a user's browser instance or app installation. Google Analytics uses the ID to identify visits across sessions that come from the same user.
Effective user ID uses the User ID if one is available; otherwise, it uses the Device ID. The Device ID is either the app instance ID for a mobile app or the client ID for a website.
Where Do You See an Effective User ID in GA4
When you create a "User Explorer" type exploration, "Effective User ID" is the field that's used to show the User ID associated with the various users
In the above screen any number that shows up with 597. is the signed-in user id, which means that's the id that's stored in our CRM system for that user. However other ID's are GA4-assigned Ids, which are either the Device ID or the Client ID, as explained above.