Why are people are able to live in deserts, mountains, and tundras?

People are able to live in deserts, mountains, and tundras because their bodies get adapted to the local climate and become immune to conditions that are not suitable for living.

Explanation: The ability to adapt to an environment allows living beings to live and develop themselves in a given environment.

Following are a few examples of adapting to the environment:

The Eskimos have managed to adapt themselves to survive in the cold higher latitudes where the availability of oxygen is very low. The bodies of the people living in the northern part of the African continent have got adapted to low intake of water.

Asmall closed hole formed in an outwash plain or moraine. forms when a  glacier retreats then remains stationary. 

Ansradar; im guessing here; good luck : )