Notification by the server to app that starting a mechanism to show a popup

Hello everybody,
I wanted to ask you something!
I am trying to trigger a notification, this is my ultimate goal, when a number reaches a certain value.
So how do I get notifications to tell me they need to notify me?
My numeric value is saved in a db on Azure and I use Firebase for notifications.
Of course I would like to activate this mechanism in an app written in Xamarin Forms and C #.
thanks everyone, hope you can help me.