This isn't exactly a mystery. Google is doing this. Several of the pieces are already in place. If you log into a Google account in Chrome, you log into Chrome

This isn't exactly a mystery. Google is doing this. Several of the pieces are already in place. If you log into a Google account in Chrome, you log into Chrome itself with that Google account. Then Google tools can use your account as an identity signal. This is already available as a feature in Google Ads and Google Analytics. The name of the feature is "Google Signals" - they've posted documentation on how it works and what it does. This identity signal works cross-domain and cross-device. Google is working to kill off other identity signals with those capabilities that would be available to competitors, such as third-party cookies. (This is also why I believe them when they say they're actively working against browser fingerprinting.)