Google Play Protect is a security system built into the Google Play Store. It protects users both offline and online. For example, every time an application is installed either from the Play Store or APK, it will scan if the application has any malware or not. Most recently, the security model on Play Protect will be updated with a new system.
Since last year, the Play Protect system has been improved so that it can scan APK codes before installation. In Play Store version 41.7.16, Google has integrated the YARA security model. This system can track multiple categories or types of malware simultaneously compared to current systems that track individually. This new update is for the safety of users when sideloading APKs without an Internet connection.
For now it is still in testing stage and it is not sure when it will be activated for all users. Usually, it will arrive in a Play Store update around a few weeks to a few months after the feature is discovered. Let's wait together.