Airplanes typically fly in the troposphere and the lower stratosphere.
The troposphere is the lowest layer of Earth’s atmosphere, extending from the surface up to about 8 to 15 kilometers (5 to 9 miles) high. This is where most of the Earth’s weather occurs, and commercial airliners usually cruise at altitudes around 10 to 12 kilometers (about 30,000 to 40,000 feet), primarily within this layer.
As for the stratosphere, this layer lies above the troposphere and extends to about 50 kilometers (31 miles) above the Earth. Some long-haul flights may occasionally enter the lower part of the stratosphere, especially in regions where the atmosphere is stable and free from turbulence.
In summary, while most flying happens in the troposphere, planes can reach the lower stratosphere for efficiency and to avoid bad weather.