As the CAD to CAM process developed, more and more files were added to the "Gerber File Set". Management of the file collection was/is a manual job. To this day, there is no fully adopted standard file collection schema.
The nature of the 3D Viewer does not allow for ignoring the purpose of each file. Like in the PCB factory, files have to be ordered in stackup, drill files assigned to specific layer spans, etc. This task can be performed automatically by using file names and a set of editable rules. The picture below shows a modified File Explorer window in action:
Alternatively, you can load your Gerber files manually, using provided File Explorer, by buttons, or by drag-and-drop.
In any case, the stackup needs additional adjustments. For example, files might be freely shifted between fields (stackup, drills, ...) or reordered by the drag-and-drop action.
Right-Click on each column and line allows adjusting parameters, like layer thickness, file function (e.g. copper or silkscreen), or drill layer span.
Please save the arranged stackup / job. This will speed up loading the board next time.