@Tomasz let’s see what we can do! Your approach sounds correct. What kind of application do you have? If it’s a single page application there is a short intro here on them Single Page Applications and as you mentioned the JavaScript Login is a good one (the guide can be found here: Auth0 JavaScript SDK Quickstarts: Login
When you played around the sample did you also follow the guide for the Quickstart guide (here: Auth0 ASP.NET Core Web API SDK Quickstarts: Authorization)?
You said you tried but without success. What happened when you tried these applications? Where did things not work. What happened and what did you expect to happen? And we can see how to fix any issues that came up.
For the ASP.NET Core API quickstart is broken down as you create/define your api in Auth0 as described here followed by defining your scopes. There’s steps to configure the sample project and how to validate the access token. and then here describes how to use your API: Auth0 ASP.NET Core Web API SDK Quickstarts: Using your API. There are also some steps on troubleshooting that includes checking the token validation, inspecting a token, and debugging.