So there’s not really a schema. Firebase is NOSQL so doesn’t require you to have a schema setup. The Google Drive sheet spits out JSON. Firebase eats the JSON straight in. No schema needed.
As for generating a Firebase access token…
Then generate a secret on that page(it might already be generated on all new Firebase projects). DB secrets are being deprecated but work for now. Transition to Firebase tokens should be easy enough.
Finally, when you set the endpoint URL on the Google Drive doc, you need to pass that secret. So…
https://<YOUR PROJECT>.firebaseio.com/<COLLECTION NAME>.json?auth=<SECRET>