When you play an animation, it doesn't know about skins. If you key attachment visibility, you are either keying 1) the visibility for an attachment not in a skin, or 2) you are keying the visibility of a skin placeholder. When an animation shows a skin placeholder, which attachment is actually shown depends on the skeleton skin. There is no way when you play a "happy" or "angry" animation to automatically change the skin. Your code at runtime that sets the animation should also set the skin.
At runtime the skeleton has two skins: 1) the "default" skin, which holds attachments that are not in a skin in the editor, and 2) the skeleton skin, or none. You don't normally change the default skin, but you may want to set (or clear) the skeleton skin. When an animation shows an attachment, it looks for it first in the skeleton skin, if any, then in the default skin.
While there is only one skeleton skin, you can easily add a bunch of skins to a single skin (using code at runtime). That is how you get the same functionality shown in the Skins view in the Spine editor. If you want to show multiple skins, create a new, empty skin and add the other skins to it.
You can run Spine with `
svor
skeleton-viewer` from the command line to start the Skeleton Viewer (you will need the latest launcher and 4.1+ version). That may give you a better idea of how things work at runtime. Click an animation plays it. There is no skeleton skin by default. Clicking a skin sets it as the skeleton skin. The Skeleton Viewer can't currently combine skins.
