I'm developing an ASP.NET Core API, and I'm trying to do the authentication and authorization part as best as I can. I'm studying OAuth 2 and OpenIDConnect (very preliminary studies at this point). But from an API developer standpoint, what can I gain from inserting an Identity Provider like Auth0 or Azure AD B2C in the process instead of just storing the user credentials using some form of cryptography?

Also, Oauth 2 seems to allow many flows, is the job of the API to be concerned with the flow of the application consuming that API? Seems a bit unreasonable. What I want is just to have a safe way to store the user credentials, and allowing the users of my API to perform authentication and authorization before consuming and manipulating resources within the other services within the API.

I understand that authentication and authorization is a sensitive topic within an application, as they deal with security concerns, and I'm planning on building an application that will deal with sensitive financial operations. That's the reason I wend after Auth0 and Azure AD B2C. But to be honest I'm having a little trouble trying to understand what Identity Providers like these will bring to the table, I know they'll bring something of importance, I just need some help to see what and why should I use them.