Google has always maintained that its own Play Store security mechanisms were strong enough to keep your devices safe from malware and other bad actors, but this was proven wrong again and again and again. Thus it makes sense that the company has announced the App Defense Alliance, a partnership with ESET, Lookout, and Zimperium that aims to stop bad apps before they can reach anyone's Android devices.
Google says that all of its hand-picked partners work in the endpoint protection industry and "offer specific products to protect mobile devices and the mobile ecosystem" by utilizing static/dynamic and machine-learning based algorithms to find bad actors, just like the California company itself does.
The partners will collaborate closely with Google and will be able to request the Play Protect scanner service to analyze potentially harmful apps they found through their own algorithms. Play Protect will send back results directly to the partners, and thanks to the integration, Google's service will also be able to request scans from the collaborating security firms. This will ensure three more pairs of eyes double-checking each others' reports before Play Store listings are greenlighted.
Just like before, Google still accepts reports of potentially malicious apps from any security researcher — it just wanted to create an additional, automated solution since it's hard to keep up with manual reports due to the sheer size of the Android ecosystem.