[djrhails]

Looks like Google has a mostly internal user id ("gaia_id"), once you've got this small identifier you can look on each Google related site for publicly available information relating to that user id.

Looks like Google is aware of it as the reference to gaia_id has been removed from Youtube pages source (Aug 2020), locked down public accessibility of photo albums (Sep 2020), and prevented connected email address leaks from Webmaster tools (Sep 2020).

[mobilio]

hmmm... what is this leak from Webmasters?

[djrhails]

Good question, found a reference to it in a related blog post: https://sector035.nl/articles/keeping-a-grip-on-google-ids and took it at face value. Can't find anything more looking further.