Here's a breakdown:
* The sky as the atmosphere: The atmosphere gradually thins out as you go higher, with no distinct boundary. It's estimated that the Karman line, 100 kilometers (62 miles) above sea level, is the point where space begins. So, you could say the sky, in this sense, ends at the Karman line.
* The sky as what we see: What we perceive as "sky" is actually the scattering of sunlight by molecules in the atmosphere. This scattering creates the blue color we associate with the sky. This effect fades gradually as you go higher, so there's no specific "end" to the blue sky.
* The sky as the background: The sky is also what we see beyond the earth's surface. This "sky" encompasses the atmosphere, outer space, and even the sun, moon, and stars. Again, there's no defined "end" to this expansive view.
Ultimately, there's no single point where the sky "starts" or "ends". It's a continuous, ever-changing expanse that extends far beyond our immediate perception.
