It's a website built to a standard that Google created, and when a site is built that way Google and iOS treat it like an app. It can be "installed" on your phone and used like a normal app.
While such access is still hard on Android (see https://news.ycombinator.com/item?id=19057673) or impossible on iOS, you on the other hand can easily call all the WinRT APIs from JavaScript if you distribute a PWA on the Microsoft Store.
See https://docs.microsoft.com/microsoft-edge/windows-runtime. There are examples of what these APIs allow you to do on https://www.pwabuilder.com/windows.
BTW, you can use https://pwa2uwp.fragara.com/ to publish a PWA to the Microsoft Store, without needing Visual Studio or even a Windows machine.