You'll probably discover that it flies somewhere in the troposphere, which is important for two reasons: the air is thin enough to minimize drag, yet it's also thick enough to produce an adequate amount of lift. For these reasons, commercial jets almost always fly in the troposphere.

Why the aircrafts do not fly in the troposphere?

Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below. The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere.

What can fly in the troposphere?

Helicopters and light aircraft typically fly in the troposphere. Light aircrafts are unable to operate at higher altitudes because they lack pressurized cabins. Therefore, they stay within a range of about 10,000 feet.