This photo shows why a baby spits up or regurgitates when on their back or in the infant car seat. You can see that the food easily can come up the esophagus. The best position would be to hold the baby upright for at least a hour after feeding to allow the food to leave the stomach
Not overfeeding a newborn infant also helps since an average newborn takes anywhere between 2-3 ounces of formula every 2-3 hours or 24 ounces a day. They have a sucking reflex and can take much more if you let them, however, their small stomachs would regurgitate the excess before it would have a chance to empty.