Just stick to low gear and let engine do the braking

D is normal drive, computer shift gear according to speed. Higher speed = higher gear. You don't want that going down hill. Downhill must have engine braking using low gear, some AT car can go down to 1, but nowadays probably only down to 2 only.

Up slope with D, computer sense not enough torque then will auto drop gear to give torque / acceleration so revs go up.

All normal unless for whatever reason you drive until red line

Uphill not necessary since computer will do the gearing work. But if you insist as it feels to give you more power then also no harm