I am not sure what kind of app you are trying to create. The demo and document I mentioned here is specific for an operation applied for SharePoint tenant level. If you just need to access one site collection, you can create console app like a time job which allows you to interact with your SharePoint site collection with a App Principal. You can reference to Kirk Evans’s blog https://blogs.msdn.microsoft.com/kaevans/2014/03/02/building-a-sharepoint-app-as-a-timer-job/ for that.

If you need to create an App to communicate to SharePoint with read permission, you can grant your app delegated permissions “Read and write items and lists in all site collections” from AAD. this permission doesn’t require admin to consent.

I hope it helps and let me know if you have more questions.