In a 3D model, how can I insert a hotspot in an object or element that is only visible during a moment of the animation because at rest it is covered by another object above?

To do this you can go to the "objects" subtab, temporarily hide the outer elements that are blocking the view of the interior, go to the hotspots subtab, assign the hotspots and then go back to the objects and make the outer parts come back to be visible.