Google Ads

Google scans their users gmail accounts and reads them, then uses keywords and phrases to choose which advertisements to show. Because I only use my gmail account for registering for websites and getting notifications about rent and airline tickets, I get some ads that are way off the mark. Hotel advertisements are apparently linked to any locations or references to airlines and Google also believes that I am interested in cooking supplies for some reason, although this database is off it makes me wonder how much about me Google could find out if they reviewed my emails, conversations, and searches.