What is and where to get the serviceAccount.json file
In a nutshell, the Firebase Service Account Key is a way for you to authenticate your app on the Google servers. This file contains multiple information about your account and your project.
HOW TO GET THE FILE
In order to get this file, let’s go to your Firebase projects page, then select your project and as shown below, go to your Project Settings.
Click on the Service accounts tab, then Generate new private key.
A file named <your-project>-firebase-adminsdk-<first-numbers-of your-key>.json get generated, and it is your ServiceAccount.json file! Rename it (or change the path) and you’re ready to go.
⚠️ Your file can be generated only 1 time, if you create a new key, it will be a different one. Although, your previous key/file will still working until you decide to delete it (click on Manage service account permissions).
I hope this quick tutorial helped you!