Propagating Holly Bush Cuttings
Why do the cuttings need to be placed in the ground upside down and which end is dipped in rooting powder?
The article you're referring to is just one method of starting holly, used by some northern gardeners. The rooting hormone is placed on the bottom end of the cuttings, where the roots will be. The cuttings are placed upside down in a hole and covered with earth to take advantage of warming effects of winter sun, and also to keep the cuttings moist. Moisture is key in starting holly. In the spring, the cuttings should be planted right side up with leafing ends in the air. For another, more common approach to starting cuttings, see this: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-root-cuttings-from-various-shrubs-bushes-and-trees.htm
Most hollies should be started from hardwood cuttings, and the do best if you set up some sort of moisture conserving system, like a plastic cover.