What not to do
This is strangest quantity selector I’ve ever seen on desktop. A perfect example of what NOT to do:
On both desktop and mobile, instead of a dropdown or only a text field, I recommend having steppers to increase and decrease the quantity, with an editable quantity field.
The chances of users buying 25 of a product are very low in most cases. But in that case, by also allowing users to tap/click on the quantity field and edit that value manually covers that scenario as well.