The site contains apk files uploaded by the community. Each time a new file is uploaded, it is scanned with 50 antivirus programs using VirusTotal service. When downloading a file, you can first click the 3 lines ("hamburger" menu) next to download button and check the antivirus report. Files that have just 1-2 warnings (out of ~50) are not removed, because even files from Google Play store sometimes contain false positives.