modifying default for date in calc

Unfortunately, hours of reading calc help do not satisfactorily answer a
simple question: I want the date in a cell to be shown as 16-Jan-20 rather
than 1/16/20 or any other option. Time and again, whatever is tried will
not save that modification to date when I push the button for insert date
in a cell.

In the Format Cells dialog, place D- MMM- YY in Format Code. Click OK. You could also use DD-MMM-YY if you want the date to always be two digits.

Dan