From Fortify's README:
Introduction
Laravel Fortify is a frontend agnostic authentication backend for Laravel. Fortify powers the registration, authentication, and two-factor authentication features of Laravel Jetstream.
In other words, Fortify does not provide any screen or UI, it just provides the authentication code infrastructure to be used by other code. And as said in the Introduction above it was built to power Laravel Jetstream.
One tip: if you are going to try to install Jetstream on your app, first be sure to have a backup of your app, if you are already using GIT you should be fine with this.
Laravel jetstream is meant to be installed on a new application. Although it is possible to install it on an existing application, the commands on its install guide are destructive, in the sense they replace any existing code, as its is assuming you would be installing on a new application.
You can install Jetstream on a blank application and then compare with your current one to see what you need to manually port from to other.