Adding web notificiations via Firebase to Android

Hi all

I have a web app that uses notifications to users that go via Firebase, and this is working well.

I have an Android App that someone developed for me, which basically houses the web site (via webView) along with some additional tools such as a Qr reader. I myself am not an app developer but a web dev.
I would like to add the notifications to the App as well - he told me he has added what he needs to do on his side, and that i need to add some code to the web site, so that that the notifications can work in the App as well.
Can anyone shed some light on this for me?

Thanks!