Jmix Backoffice UI allows you to develop the rich web UI using Java and XML. It is based on the Vaadin web framework.
The Backoffice UI technology has a limited scalability: the backend side of the UI is stateful and consumes the amount of resources proportional to the number of concurrent user sessions. So Backoffice UI is suitable when you have a predictable and not very high (up to tens of thousands) number of users working with the system at the same time.
For a web-scale project and full freedom of UI customization, consider creating a frontend application using a traditional frontend technology. It can work with the Jmix backend through the REST API.