The first step of oAuth is to redirect/send the user to that URL in a browser in order for the logged in Twitch user to allow the link between your application and that Twitch account.
Then after allowing the link, they are redirected to your redirect URL with a ?code
That ?code is then exchanged for a token to use with the API
This is a “one page” PHP example describing the workflow https://github.com/BarryCarlyon/twitch_misc/blob/main/authentication/user_access_generator/php/index.php
Step 1) Redirect the user to Twitch
Step 2) They accept (or decline) access to the ClientID to their account
Step 3) they come back to your site with a ?code or error message
Step 4) if code exhcnage the code for a token (and refresh token)
And that is User oAuth in a nutshell