>>264403072
>>264403992
My man IK bones are just tools used to help pose a rig for doing animations and such. Basically they compound movements in a more natural way for posing so you don't need to tweak every single bone. For example say you wanted to animate a walking sequence, you wouldn't want to have to tweak every bone in the chain for every frame as it would be hard to make it look natural. So instead they'll parent an IK bone they can slide around that will smoothly move several of them to the right position.
That said you do not need this for any model you make unless you're planning on making your own animations. All you need is the humanoid rig and you just need to take care to use the same naming convention VRchat uses. I think these are in the guides but if you can't find anything let me know and I'll dig it up.