Help Understanding OAuth

This gives you a Server Token, which is only useful to give you a higher request rate for non user API requests.

You take the code and perform step three of the oAuth dance, you exchange the code for an access token, as documented:

3) On your server, get an access token by making this request:

POST https://id.twitch.tv/oauth2/token
    ?client_id=<your client ID>
    &client_secret=<your client secret>
    &code=<authorization code received above>
    &grant_type=authorization_code
    &redirect_uri=<your registered redirect URI>

After obtainin an access token via step 3 of hte oAuth dance you can make an authenticated request to

https://api.twitch.tv/helix/users

as documented:

Thats entirely up to you and how you wish to handle oAuth/third party service logins/session management.