Hi @kartrabby,
Granting access via Azure AD App-Only covers how to register an app with app-only permissions in Azure AD and how to use that app to interact with SharePoint using PnP PowerShell and the SharePoint PnP Sites Core library. Unfortunately, this resource doesn't cover how to use the app with the "plain" REST API. The basic steps to do this would be to use the Microsoft Authentication Library (MSAL) library to get an access token and then make REST API calls with that token included in the headers of the request.
You could refer to the nice article
Testing client certificate authentication to Azure API Management with Postman
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