You would generate/create the Application yourself, which gives you a clientID and Secret.
Then the streamer Evanbear1 goes to your website, clicks a login with Twitch button then grants your application access to their account.
Then you get a access_token (and refresh_token), and can use those with your ClientID to access the API on their behalf.
You’ll need the refresh token so you can automate getting a new access token when the access token expires. (So avoid implict auth, as otherwise you’ll need the streamer to grant access again in two months)