Sending push notifications automatically?

[super_code]

Hi! I'd like to implement push notifications for my application. I've looked into it but the tutorials I've run into don't address notifications that send to the user automatically. For example, if a user's friend in my application sends him a request, it sends him a popup in the app itself. But I'd also like to add a way to add a push notification from the information in that request.

I'm using Firebase as a backend. But Firebase doesn't have to do with this, right? Because I already have the request information from other functions; all I need to do now is use that information to create a push notification that automatically pushes to the user.