But I guess most of the blender options don't work with assimp. I tried to use "copy rotation", "copy location", in order to copy the rotation of the forearm into the rotation of the hand, the animation works perfectly in Blender, because when I rotate the forearm, it rotates the hand bone too, but when I export it to collada and load it with assimp, it fails hard when using assimp.
Remember that IK, Drivers and constraints are often not supported when exporting and some times your engine won't have options like these. Most exporters will attempt to convert it into a animation if it can, check your exporters and game engine to see what is supported.
Even with .fbx files imported to Unreal, the IK needs to be rebuilt because it doesn't agree with Blender. It's often better to do these things in the game engine.
the hand is cut from the arm when I move it. What is the simplest stupidest conclusion that a primitive mind like mine can come up with? Oh no, the bones were disconnected!!! That's why I just started Blender, loaded the project, clicked the hand bone, then shift+click the forearm bone and I just pressed Ctrl+P and clicked CONNECT to connect the hand bone with the forearm bone and now everything works perfectly fine. Can you believe this? I spent days on this, WHOLE DAYS
Good to see that you fixed it, this should also be useful to other game developers with the same problem.
How are you building your rig that the bones are disconnected?
The way Blender expects you to work is that you use Shift-A to add a Armature, then go to edit mode. Now when you want to add a bone don't copy the first one, use the E-key to extrude a new bone, it will be connected. Ctrl+LeftClick will also extrude a new bone, using the pointer as the target.
The W-key will give you a specials menu where you can divide a bone, useful for spines.
Never scale a bone in edit mode and always turn off the "Inherit Scale" option(It's broken, It's been reported).
When I first started with Blender I found these rules hard to follow because I used other 3D software first, however once you start using it you will understand it. Because rigs also "break up" when working with them it's a good idea to do a stress test before exporting.