The Aps.cer is needed to create the firebase_push.p12 certificate which is uploaded to firebase. The steps are in the link you referenced.
The other production certificate is for your app in release mode.
Using this method you need to recreate the Firebase certificate once a year.
Alternatively, you can use the following method to create the firebase certificate once only, which is also mentioned in the link you referenced.
Until now whenever I wanted to add Push notifications to my apps, I have followed the steps outlined in Erel's tutorial https://www.b4x.com/android/forum/threads/firebasenotifications-push-messages-server-not-required.68645/ This works great except that each year I need to update the APN...