PWA apps are web apps which means they are usually web pages. They could provide users native-like experiences using modern web technologies (Service Worker, Fetch networking, Cache API, Push notifications, Web App Manifest). UWP apps are traditional native apps.
>> Are MS PWAs different from Google PWAs? Ofcourse other than, the framework that it was built upon.
MS PWAs using Edge as its engine and Google PWAs will use chrome v8 engine.
>> How to deploy a MS PWA so it is available across devices and OSs?
PWA apps could be hosted on your servers and can be updated without issuing new updates to an app store. So that it is available across devices and Oss. But for Windows 10, PWA apps is more easy as MS PWAs could be uploaded to the Microsoft store. This means a user will be able to engage fully with an installed PWA—from discovery, to installation, to execution—without ever opening the browser.