Every piece of armor with warmth generates heat (it’s not really insulation, just +heat).
Some mutations cause heat or loss of heat.
Being hurt, hungry, tired or having common cold decreases heat.
Standing outside cools you down based on wind power (mostly random) and you being wet.
Sitting inside a car (has to be on inside tile and vehicle has to have batteries), wearing a powered power armor or using a climate controller CBM move current heat status one step to comfortable (scorching -> very hot, chilly -> comfortable etc.).