What IK does is calculate how things should act when linked. So if a hand moves to press a button, how should the arm move.
Now any two characters that are the same height,size and bone count can already share animations, Just copy and paste animations in your 3D editor.
Re-targeting is needed for the objects that don't share these 3 properties.
The way animation re-targeting works is the difference from one characters stance and the second stance is calculated.
This is simple: Rig2Foot.position - Rig1FootPosition = offset between feet.
With this info you can use IK, to generate the new animation. This video shows the results:
This video teaches the first steps for 2D:
For 3D it is already in Godot you just need to figure out how to use it.