Hi @DavyGevaert,
Welcome to the Auth0 Community! 
The new Blazor model is actually a bit tricky for authentication. There is a thread on this topic in this forum.
I’m planning on digging into it and writing an article in the next few weeks.
To quickly answer to your question about OIDC and OAuth2, OIDC is related to user authentication, so . AddOpenIdConnect adds support for authentication.
However, OIDC is built on top of OAuth2, which is related to delegated authorization. This means that using OIDC you can also request an additional token (access token) that lets you access OAuth2-protected APIs.
The following are introductory articles about OIDC and OAuth2:
I hope this can help.