Firebase for push notification

Hi, I have a question about push notifications with the Mobile Engagement module. I have been reading all the documentation in Marketo Developers about the push notifications and don't find what I need for Firebase.

1. I understand we necessarily need to use Firebase in order to send push notifiations. Is anyone using a different system? Is there any alternative to Firebase?

2. What is the architecture between Marketo-the app-Firebase? Where are push messages stored? Are they stored in Firebase? I'd like to understand the architecture. Are messages encoded?

3. In order to get a plan in Firebase, you need to estimate the volumes of the notifications you are going to send. Since we don't count on this yet, do you have an estimate about the volumes? We need to get a plan for Firebase but don't know which one to take. Don't know the volumes of the messages sent via Marketo.

Thank you,

Sara