for low kicks, there's a nerve cluster on the outside of the thigh just above the knee and an exposed ligament, hitting the side of the actual knee can fuck up your shin, especially if they check out, and going higher you run into a lot of muscle mass which can better absorb the blow.
Thiago Alves - Low Kick - YouTube
pretty perfect placement there
knees should be targeted either right at the diaphragm or the floating ribs, midkicks the same, floating ribs and livers are nice, if you go low you'll just bang into hip and too high you'll get more muscle mass, less exposed organs, thicker ribs, and it will be harder to land through a guard.
remember when Jardine fought Liddell he landed like 80 midkicks but very few of them were properly placed which is why Chuck was able to eat them all without much problem.
elbows are super varied but if you're slashing downwards try and split the guard right down the middle to connect with the nose, a horizontal one should crash into the jawline or cheekbones. nose if you can but the angle usually won't be there.
teeps right at the diaphragm are your best bet, if you're lucky they'll be taking a gulp of air and you'll knock the wind out of them.
there's leeway with any of these but those are generally the areas you want to target / protect, which is why it's so damn important not to raise your guard too high which almost all beginners do. a good muay thai guy will devastate your body if your elbows get too high to protect that about six or seven inches between hip bone and the weaker ribs.