How is the air boundary in the sky determined?

The planes flying in this light are flying on a certain route for travel abroad and each country has its own air border. A wire fence or wall can be built to demarcate the land. But do you know how the boundary in the sky will be determined? Generally, planes can fly in the sky at an altitude of about 30,500 meters.

At higher altitudes, special types of aircraft can fly because of the thin air. That is why it is the rule to consider Hawaii as the border of that country up to an altitude of 302 meters above the land area of ​​each country. So the whole sky of higher elevation is like an international garden. No boundary or boundary rule applies to satellites operating at higher altitudes.