Well they operate together as a lever of the distance of the hypotenuse of the stem length and bar width. (Any fore and aft distance o the bar gets added to or subtracted from the stem length.) So, say the bars are 42 cm and stem 10. Straight bars. Hypotenuse will be 23.3 cm. (Bar width from stem was 21cm,( So you can see the 10 cm stem had s9ome effect but not a whole lot.

Now this is if you are spinning the handlebars like they are a near horizontal version of the wheel of a ship. If your input is just push/pull, then your input is parallel to the stem and its length has no effect at all. So the stem contributes between zero and 11% to the leverage (for this pretty typical road bike).