Follow below mentioned steps to create Firebase JSON file :
Step 1 : Go to Firebase and Sign in to your account.
Step 2 : Click on Get Started
Step 3 : Click on Create a Project
Step 4 : Enter your project name
Step 5 : Select the parent resource and tick mark the checkbox to accept terms and conditions.
Step 6 : Click on Continue
Step 7 : Enable Google Analytics for your project and click on Continue
Step 8 : Select your desired Analytics location, and tick mark the checkboxes to accept terms and conditions
Step 9 : Click on Create Project
Step 10 : Click on Continue
Step 11 : Click on Android icon
Step 12 : Add Firebase to your android app
Step 12 : Click on Register app
Step 13 : Download the google-services.json file on your system and Click on Next
Step 14 : Click on Next
Step 15 : Click on Skip this Step
Step 16 : You will be redirected to your console dashboard and Now go to Appy pie google class feature.
Step 17 : And, upload the downloaded JSON File here.