Regardless, the last time I dug into this topic I ended up feeling the same. The web is littered with articles that scratch the surface and only cover the basics. They often leave out the details, which IME ended up making things more difficult to understand. What was the most helpful, as you said, was to follow the RFCs and the OIDC spec directly.
What might also be useful, if you are implementing an auth server, is to look at existing implementations. Duende IdentityServer (https://github.com/DuendeSoftware/products/tree/main/identit...) is the most widely-used one in the .NET space.