Google APIs on www.googleapis.com

In Google Cloud Console Credentials, create an API key for your project, then restrict it as needed. Google’s auth docs link to Manage API keys and Use API keys to access APIs.

Create OAuth client credentials in Google Cloud Console Credentials, configure the consent screen if prompted, then run the appropriate OAuth flow for the API scopes listed in that API’s Discovery Document. For service-to-service access, Google’s REST auth docs point to Authenticate for using REST and related Application Default Credentials guidance under Authentication methods.