I'm wondering about the notifications supported by ML2, if there's a tutorial on using the system notifications vs. ML2 notifications. I still haven't tested the "dialog" feature supported by MRTK to see if it works with ML2. If there's a API to push notifications to the users and show them customized notifications.
We actually use the Android Notifications system under the hood but with some caveats. You can read up on the interface here Notification | Android Developers and we partially support API level 29 currently.
As for the caveats we only support the title, summary, and description fields plus payloads that can currently exist are the Action(s) (i.e the buttons, which emit a PendingIntent) and an embedded image. We don't have the options for push providers etc but this should allow you to emit a simple notification with a pendingIntent or starting another app.
As far as MRTK Dialogs I don't believe those outbound to a separate notification service but are rather rendered in process by your application, they are essentially prefabs. This means you should have a lot of control there as long as your app is in the foreground and rendering to the user when using them.