Notes:

For date and time fields, we have to use the QgsDateTime widget instead of the Qt native one, as it supports Null values, too.

In the UI Designer, you cannot name a object "class", it will be names class_2. But it can be renamed "class" manually in the XML file. (Why?)

It is to investigate further how how function/usage arrays (that become list in the drag-&-drop form) can be mapped to checkable comboboxes in the UI file.
Is it possible at all?
Which workaround is viable as solution?
https://gis.stackexchange.com/questions/354462/display-a-list-field-in-a-custom-form-with-qgis-and-qt-designer
Natively, the "LIST" field proposed by QGIS causes a crash when loading a UI file.
Which object is best suitable for a UI file?