On the normal web, a user might visit
https://gmail.com/, their browser
will download the Gmail user interface which then runs in their browser and connects back to the Gmail servers.
On Freenet the user interface is downloaded from a Freenet contract, and it interacts with contracts and delegates by sending messages through the Freenet kernel.
Because UIs run in a web browser, they can be built using any web framework, such as React, Angular, Vue.js, Bootstrap, and so on.