Cool video Shiu! To make it clear what is being shown, he has an animation running and he switches between two completely different skins. The skeleton and animation are the same, showing that you can reuse all that work for multiple characters. This alone may seem trivial and for naive, limited usage it is, but there are some important details in how Spine handles this.
Imagine you want your goblin to blink during an animation, so you have the animation change the image for the eyes briefly to closed eyes, then back to open. If you go and use the male goblin eyes in the animation, when you change to the female goblin skin it would show the male goblin eyes. I’ll leave the details to the documentation, but Spine handles this properly, allowing you to reuse all animations with completely different skins, even those animations that need to swap images.
Another thing Shiu showed in the video is changing to setup mode, modifying the bind pose, then going back to the animation. He made the bind pose look up, then showed that when the animation is applied, the character is looking up. This allows you to do procedural animation, eg having the player look at the mouse location.
I think it would be best to wait to model your engine’s runtime off one of the official Spine runtimes. It’s a bit tricky to get exactly right.