The better resource to know about oAuth2 is IETF, and about OpenID Connect is openid.net.
IdentityServer is an implementation of OAuth2 and OpenID so it's documentation will not cover the basics that related to OAuth2 and OpenID.
To understand first about scopes you should have a clear understanding about clients.
Client: Models an OpenID Connect or OAuth2 clients (not your client application) and it should have a clear flow for example you defined a client which uses implicit flow.
The flow is the way that you should follow in order to get the needed data for example access_token and id_token.
All flows can be used with any cases but there are recommended flows for every case.
For example, up until now, it was recommended that you use implicit flow with native and javascript clients. Though, recently this has been changed to Authorization Code with PKCE. See Identity Server's blog post on this change to IETF/OpenID recommendations
Scopes: Models an OpenID Connect (Identity scopes like email, given_name etc.) or OAuth2 (Resource scopes like your WebApi that you want to protect it's data) scopes.
You can think about scopes as intent of the client, for example: The Client ask you to use your resource owner to grant me access to your openid scopes > given_name, email & prefered_username and your OAuth2 scope > WebApi.
For full understanding:
1- Pluralsight - Building and Securing a RESTful API for Multiple Clients in ASP.NET
2- Pluralsight - OAuth2 and OpenID Connect Strategies for Angular and ASP.NET