Im having some problem when rotating bones because some deformations occur (shown in the pictures)
You need to improve skinning so that the wrist influences more vertices on the forearm or have a twist bone like the UE4 Mannequin. Having a twist bone will produce better results and will be more flexible when animating but if your skeleton doesn’t already have a twist bone then it can be time consuming to add one to the skeleton and then re-skin. Adding a bone will also break animations which will then require re-targeting.
I haven’t done any character rigging, all the characters I have tried are or from mixamo or the ue4 store but even with the paragon characters I get the same issue. I even tried using a free character meant for VRChat but I got the same issue
Rotating individual bones in editor is not the best way to create animations. There are probably twist bones which are designed to prevent this issue. You need to have a proper rig for your character which can be done in the editor.
When you have a proper rig it will affect the twist bone when you rotate the wrist and will look a lot better.