The Earth's Atmospheric Layer

What counts as the earth's atmosphere when you were in a plane above the sky? Are you technically in space? When does it become space? There's like different layers of the earth's Atmosphere. And where are you when you're flying in a plane? You're not even in any of that, I think. Do we think the pressure increases or decreases when you're going through the earth'smosphere? If a plane wanted to, could it exit the stratosphere or does it not have enough power to do that? Why does a plane need to be so high? Like why does it have to reach a certain height? How do they decide how high it was going to be