"We may use the information we collect across our services and across your devices for the purposes described above. Depending on your account settings, your activity on other sites and apps may be associated with your personal information in order to improve Google's services and the ads delivered by Google."
Google links user activity across all its services (Search, YouTube, Gmail, Maps, etc.), across devices, and from third-party sites and apps that use Google services, building a comprehensive cross-platform profile.