Hi @lakeside_east, welcome to Thunkable! 
Be sure to check out our posts about How to ask Great Questions v2.0 and our Community Guidelines as you get started.
Alerts and Push Notifications are two separate features. Alerts only work in app, while push notifications can be sent to a user regardless of whether they are using the app or not. If you are going to incorporate Push Notifications into your app, you will need to create a OneSignal account as Push Notifications are handled through their service. Firebase accounts are only required if you are setting up Push Notifications on Android devices.