Google Play Protect is just about two years old, as it was introduced at I/O 2017. The tool scans all applications installed on your phone for identified malware— whether they are from the Play Store or from a sideloaded APK. At Google I/O 2019, it was revealed that Play Protect now scans 50 billion applications each day.
The number was announced as part of the focus on privacy and security during today's keynote presentations. Play Protect has seen a number of improvements since it was introduced, like a new interface added earlier this year, though it mostly remains a silent and unobtrusive component to most people.