The requirements on the field are not the same as on a desktop. The screen is smaller, the input devices are different and the tasks are different.
QField aims to help users to perform the tasks they need to do without cluttering the user interface. This means, that only tasks which need to be done on the field are availble from the interface. Everything else is not.
This means that everything like layer styling, form definitions and other project setup steps should be done on a computer with QGIS installed first.
QField is based on QGIS. It is not a rebuild of QGIS it really does use QGIS libraries. The rendering engine is exactly the same as in QGIS for desktop and your project will therefore look exactly the same on your mobile device as it does on your computer.
If something is already available as a configuration option in a QGIS project, it should not be re-invented. QField therefore uses the same edit widgets as QGIS desktop does. If a project is already configured for the desktop, it should just run on mobile as well.
Remember, this is just the concept. This is what we have in mind when we develop QField. It does not mean that it is already completely there yet.
QField is built around modes. Modes are similar to a map tool in QGIS desktop. A mode defines the task which a user is currently doing. Either a user is browsing through the data or she is digitizing something new.