Typical holsters can be positioned in such a way that they are concealed or open carry.
An example are belt holsters (such as what many uniformed police use), paddle holsters, in-waistband holsters, and shoulder holsters. If you can hide the gun's presence (TOTALLY) then it is concealed. This can be done by pulling a shirt over the gun and holster, a vest, coat of any type, etc. I have a paddle holster, a belt holster, an in waitband holster and a shoulder holster. Of all of those, the paddle holster is hardest to conceal (or keep concealed) because it is bulky, but it is effective, there is no retention on mine so I don't use it for carry typically in public.
If ANY part of the gun is visible, then it is openly carried. In North Carolina, concealed carry means completely unseen, this does not include imprinting (seeing the shape of a gun or holster through clothing); so if the gun is concealed but I can still see an imprint of any kind it is still considered concealed. If the butt of the gun is visible only for example, that is openly carried. I may get an argument against this but there really is no partially concealed in North Carolina. NC clearly defines a concealed weapon. Anything less than that definition is an openly carried weapon (this is important to know, not just for firearms, but for other weapons that are not allowed to be concealed at all under any circumstance such as stun guns and knives).
Perhaps the only open-carry holster that would typically not be concealable is a drop leg holster (typically seen in use by SWAT-style law enforcement and military), but it can be concealed by means of a long coat.
A holster type that is typically going to be concealed are belly-band holsters. Unless you are shirtless or like showing your belly off, this will automatically be covered up by the shirt, wearing it on the outside will make you look goofy and is perhaps impractical. Other holsters that are completely concealed include waist-fanny packs, those made for going in a coat pocket or pants/shorts pocket (so the gun is not floating around and is "fixed" making drawing more reliable and practical.
You also have ankle holsters which, unless you're wearing something that is above the ankles (or high-waters that ride up when you walk or run), will be concealed. There are also crotch holsters which, yes, go over the pelvic region, and the ones I am referring to work similar to the belly-band holster except set lower. Sometimes, these crotch holsters may allow you to carry your gun openly but typically that is not the smartest way to use it.
I hope that answers your question(s).