How do games change the meshes at runtime?

Right, I'm making a game and I'm not sure how I'm going to do this:
On Skyrim (for example; there are thousands of others), when you equip a helmet, it appears on your character's mesh. How do they do that? Do they modify the mesh, or simply render the helmet at a location which makes it look like it is on their head, or what?
If you know I'll be very thankful if you can tell me how.
Well I might give you some examples in other games. In Warcraft 3 models have "junction points". You can bind some other model to it and that model will be rendered alongside unit model. In World of Warcraft a particular mesh (body or head for example) will be replaced depending on character equipment AFAIR.

Minecraft render armor over body, and Team Fortress 2 does the same for most hats and accessories. Also weapons in TF2 has their own model which is rendered separately from player body.
Okay thanks
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