I'm trying to work with an outside company who is developing an app for my company. Its cross-platform Android/iOS, and being created in Xamarin. I'm maintaining the back-end API and I was curious what push notification libraries/services people are using? I see AWS SNS, Firebase, and some old PushSharp stuff (though that project seems abandoned).

A couple things I don't really understand from the workflow, as well:

1. The mobile app has to pass my API a registration or device ID at some point so I can send push notifications to it. How do I know which one to send to when a user is logged into my app on multiple devices? if users are always user@email.com how can I say "send to all device ID's for this user?" Do I just have to store and loop through multiple device IDs on my end?

2. If a user uninstalls the app, how do I know that happened? How would I know to delete their device/registration ID from my system?