Google publishes its Android Security & Privacy 2018 Year in Review March 30, 2019 https://betanews.com/2019/03/30/google-android-security-and-privacy-2018-year-in-review/ Android Security & Privacy 2018 Year In Review (PDF - 2.59 MB): https://source.android.com/security/reports/Google_Android_Security_2018_Report_Final.pdf
Quote from that betanews article: And how exactly knows Google that? How does Google know about the percentage of apps installed from outside Google Play? What are Android phones telling Google?