In the event they do, the third-party software adds a Google Sign In flow to their software, whereas their users can press a call-to-action for signing in with Google, which would trigger an opening of a separate Google-owned domain in a new min-browser window that the third-party software cannot access (and therefore not harvest information from). This min-window then sends the user back to the third-party software domain upon completion with an authentication token - which could be in the form of a URL query string, an HTTP method, a cookie, or even collection of arbitrary browser information for fingerprinting. The third-party site then sends that authentication token back to Google via their API, and Google sends back ONLY what that authentication token is permitted to grant access to - which would not be Google credentials.