When you sign in using Google on any site, Google sends back the following data to the site:
Your name (Display name)
Your email ID
Your profile picture (Actually it's a url to your profile picture)
If a site needs any data other than name, email and profile picture then the user is prompted to grant the permission for the site to access the requested ...
You will find all the websites that give Facebook your activity.
Amazon, google and everyone that you use games etc will give your info to Facebook. That’s why when you search on amazon or google they appear in your feed
You provide a phone number (and recently, a "REAL" number: can't use Google Voice or other VoIP numbers), and an e-mail to sign up.
Whoever has that e-mail OR phone number saved to either their phone's contacts or facebook's contacts (YOU don't have to be on FB for your friends to save you on that app's/site's contacts) AND allowed Venmo to access that ...