Buddha is sometimes called the Sky of the Heaven of Ultimate Truth, because he realized the highest truth. As such, it also has the connotation of equality or undifferentiation. Our understanding of space has increased with the advance of science and technology, but in Buddha's time, over 2,000 years ago, people had a very limited understanding of the physical world. Nonetheless, Buddha and others of his time spoke of an extremely large cosmos. Their description is, in fact, in accordance with the idea of space that we have now.
Space is limitless, and all kinds of things can fly through the sky without any obstructions.
Everything changes, and we cannot find, in any phenomenon, characteristics which are intrinsic and do not change. For example, water can be a liquid, a solid, or a gas. It can be fresh, then salty, then fresh again, and it can separate into hydrogen and oxygen in our bodies and become part of us. Things do not have any unchanging, intrinsic nature. In the same way, there is nothing that does not change in ourselves.