What is the name of the uppermost layer of the atmosphere? why does it feel cold there, even though the temperature can be very high?

The uppermost layer of the atmosphere is the exosphere and it feels cold there because the majority of the Earth is heated by the molten core of the Earth and not the sun, so since the exosphere is the farthest from the core it can be very cold.
Hope this helps!
(and sorry for any typos)