Tools for Qt Quick UIs
Qt provides a tool for designing Qt Quick UIs called Qt Design Studio. Most of the functionality of the tool is integrated into a developer tool called Qt Creator.
You need to separately select Qt Design Studio in the list of modules and tools presented to you by the Qt installer, whereas Qt Creator is installed by default when you install Qt. You can run both tools in parallel without one interfering with the other.
If you only design UIs and don't need the developer tools, you can install the stand-alone Qt Design Studio package. It contains all the Qt modules and tools you need to design UIs and preview them on the desktop. The Qt Quick code is automatically generated for you, and you can deliver it to developers for implementing the application.