That menu in a Numbers cell is referred to as a Pop-up menu Format. You can set that format in the Cells Inspector.
You edit the content of the list in the Cells Inpsector. You can do it for one cell and then Copy/Paste to others, or you can do manu at once with a multiple cell selection.
It would be easier if we just gave you teh reference in the user manual instead of reproducing it here...
Page 99 is titled: "Using a Checkbox, Slider, Stepper, or Pop-Up Menu in Table Cells"