In StackOverflow's case, login happens using Oauth2 by way of accounts.google.com, meaning your google login creds don't go through StackOverflow's servers - Go

In StackOverflow's case, login happens using Oauth2 by way of accounts.google.com, meaning your google login creds don't go through StackOverflow's servers - Google acts as a trusted 3rd party and verifies that you are who you say you are, and tells StackOverflow.