Cloud Firestore REST API

Create or use a Google Cloud project in the Google Cloud Console, enable Firestore access for that project, and obtain an OAuth 2.0 access token using Google's standard application or user credential flows. For server-to-server use, the easiest path is usually to create a service account in the Google Cloud Console and use Google Application Default Credentials or service-account-based OAuth to mint a bearer token. Firestore docs state REST requests accept a Google Identity OAuth 2.0 token in the Authorization: Bearer header.

Create a Firebase project in the Firebase console, enable Firebase Authentication, and sign a user in through Firebase Auth to obtain that user's ID token. Firestore docs state REST requests can use a Firebase Authentication ID token as a bearer token in the Authorization: Bearer header; the request is then evaluated under that signed-in user's permissions.