First of all, do not use unions for large objects as they are heavy on performance and large unions can cause many problems, you can simply do this by making a sphere in blender and using the invert faces feature to change what the players see, so from the outside it won’t be seen, but from the inside you can see it, you can set the mesh to be neon black so that it looks just like the image you attached. To move the mesh, either track the position of the player or attach the mesh part to the player. This is the easy way.
If you want to be harsh on yourself, and actually want to have a whole system for it, you can just create a chunk system, where you only load and render those parts inside the player’s current chunk, but in a bit more professional way since you don’t want to render a chunk, you want to render a chunk around the player.