Check out example codes for "The Google OAuth Review Process". It will help you in understanding the concepts better.

Code Example 1

These are some url's that will help you in your verification process :
1. https://aquoid.com/plugins/photonic/google-photos/#api-key
2. https://medium.com/@crspybits/the-google-oauth-review-process-9d1b05f53aea

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