How to select or autofill all distinct elements from a column in Numbers?
I can't do it in Numbers. Numbers  has some interesting layout tools,but its utterly pathetic for anything signficant.

If you export as an excel sheet, save to google drive, then open in google sheets it's easy.

Cut and paste seems to work between Numbers and Google sheets too.  Values only, AFAIK.

To get a list of unique categories:
In Sheet 2 enter the following formula at the top of the column: =Unique(sort(Sheet1!B:B))  This will autofill down.
Now if you want them as columns instead:

Create a third sheet.
in A1 enter the formula =Transpose(Sheet1:A:A)

If this happens a lot, leave it in place.

If it's a oneshot, then select, copy, paste special, paste values only.