You create a website
You send the user to the website
The user clicks “sign in with Twitch”
The user is sent to twitch
The user checks they are logged into the right twitch account and accepts or declines the link
The user comes back to your site with a ?code
Exchange the ?code for an access token/refresh token
For example I could say “hey I need an access token from you please visit my website Twitch Implicit Auth Example to provide this”
And away we go.
This is a link to the implicit oAuth flow. Which differs from the flow described above. But should illistrate what I mean by a “website” and the oAuth loop.
There is also a farily extension guide with some Twitch Specific stuff for oAuth here Authenticating with the Twitch API
But this is just industry standard oAuth. Once you know how it works you can get tokens for a lot of different services jsut feed it different URL’s