I find this question very interesting :) So, Earth’s atmosphere doesn’t just stop—it fades away gradually into space. Gravity holds the air close to the planet but as you go higher, the pull gets weaker and the air thins out. Eventually, it blends seamlessly into the vastness of space, making it impossible to pinpoint an exact edge of the atmosphere. Instead, it’s more like a soft gradient that transitions from sky to space.

There's more to it : https://www.wtamu.edu/~cbaird/sq/mobile/2024/06/06/where-does-the-atmosphere-end-where-is-the-edge-of-space/