OAuth 2.0 token introspection questions

I'm trying to understand token introspection as I need to implement token introspection for OAuth 2.0. But authentication is so hard to understand... So I've got a couple of questions:

(1) As far as I understand, a post-request (with the access token) is sent to the IP. This then returns whether the access token is valid or not, as well as further information such as the user name.
This looks like the official spec to me: https://www.rfc-editor.org/rfc/rfc7662 it says a post request is needed to validate the access token. Did I understand that correctly

(2) This looks like the corresponding dependency, is it? using using IdentityModel.AspNetCore.OAuth2Introspection
Whats the difference between Microsoft.AspNetCore.Authentication.JwtBearer?

(3) According to the spec only the access token is requied (https://www.rfc-editor.org/rfc/rfc7662#section-2.1)

This example does not pass it but the clientid and the clientsecret:
https://github.com/IdentityModel/IdentityModel.AspNetCore.OAuth2Introspection

services.AddAuthentication(OAuth2IntrospectionDefaults.AuthenticationScheme)
    .AddOAuth2Introspection(options =>
    {
        options.Authority = "https://base_address_of_token_service";

        options.ClientId = "client_id_for_introspection_endpoint";
        options.ClientSecret = "client_secret_for_introspection_endpoint";
    });

Why not?

(4) If I set the config right, .Net Core will do the post-request (for every incoming request to my api) for me automatically, right? (I also added the [Authorize]-Attribute)

(5) How can I get the user context?

(6) I implemented a small example but get a 500. I do not see any error output. How can I log the errors?

The following are some of my understanding and opinions:

(1) The token introspection endpoint needs to be able to return information about a token, so you will most likely build it in the same place that the token endpoint lives. The request will be a POST request containing just a parameter named "token".

(2) JWTs are typically validated locally on the resource server.

It's a technical detail that IdentityServer can also validate JWTs at the introspection endpoint. That could be used e.g. when the resource server does not have an appropriate JWT library (and you don't want to store reference tokens on the IS side).

(3) In my opinion, this appears to be a configuration requesting an access token. Access tokens are the thing that applications use to make API requests on behalf of a user. The access token represents the authorization of a specific application to access specific parts of a user’s data. The Client Credentials grant is used when applications request an access token to access their own resources, not on behalf of a user. Please check this link: Access Tokens.

(4) If you add the [Authorize] attribute to the method that requires authorization, if the configuration is correct, the .Net Core App will automatically request authorization from the authorization server with your information.

(5) Do you want to access end user context on resource server? Maybe this link can help you.

(6) For logging error messages, please refer to this link: How do I log authorization attempts in .net core.

Hope this can help you.