What are some ways to securing web api with authorization in Azure

Question

I am developing an Web API .net core and hosting it in Azure as we are migrating to Azure. To secure the web API and for Authorization (Protect a web API backend in Azure API Management using OAuth 2.0 authorization with Azure Active Directory). But I have some questions as following below:

Question 1. if I protect a web API backend in Azure API Management using OAuth 2.0 authorization with Azure Active Directory then if we want to expose the Api to the third party outside of the organization then would it work?

Questions 2 Can we protect an Api that is hosting in OAuth without Active Directory.

Question 3. What is securing an api with Microsoft Identity vs OAuth. It is confusing to me why should I not use Microsoft identity over OAuth for Authorization. Is it something new that came out from Microsoft?

https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-v2-aspnet-core-web-api

Thanks