Any idea how Netlify does it? There I can only select from the repos I have granted access to, and if I want to add a new one I click "Configure the Netlify app

Any idea how Netlify does it? There I can only select from the repos I have granted access to, and if I want to add a new one I click "Configure the Netlify app on GitHub", which opens a window where I can choose which repos to allow access to. Always wondered how that works.