I've written the initial pass of Fortify documentation which documents all of the backend features that were available via the laravel/ui package:
Fortify is a headless authentication backend for Laravel. Think of it as the backend authentication logic that used to be in laravel/ui but without any specific frontend attached to it. You bring your own frontend. However, a great example of how to implement the templates is not only documented but is available for review with the Blade based authentication views in Jetstream: [https://github.com/laravel/jetstream/tree/1.x/stubs/resources/views/auth](https://github.com/laravel/jetstream/tree/1.x/stubs/resources/views/auth)
In summary, you can use Fortify as a backend for any frontend stack you want. Someone could even publish frontends as a package (like Jetstream!) on any stack you want, such as Bootstrap.css, etc.