We're not using hitboxes anymore. For those that don't know what hitboxes are.. they're this.
See those boxes? Well when you shoot a character in TF2, it traces a line forward from your eyes until it hits something. Characters in TF2 are made up of thousands of triangles, so it would be stupidly slow to test against all of those. So they stick some boxes where the head is, where the arms and legs are, where the torso is, and test against those instead. And this works great.
But it's 2014 and putting hitboxes on stuff is hard work. So we're using a skinned mesh collider. So check this naked guy out.
We already have a really low poly version of the player character thanks to LOD'ing.
So we can use that for collision!
It's a skinned mesh, so each vertex is weighted to up to 4 bones. When the mesh is 'shot', we use the triangle ID to get the vertices, then we use the vertices to find the bone. The bones have multiple attributes. For example, the head and neck bones have multipliers, so that headshots to more damage (this is indicated by the red spheres in this picture).
This gives us a lot of benefits moving forward beyond just being more accurate to the render mesh. For example, if we were to add a helmet on the player we could make it so that getting shot in the face still killed them. This is something that's not impossible with hitboxes, but it would be kind of clumsy.