Every chip turns electricity into heat while it works. Devices are designed to move that heat away fast enough to keep the chip in a safe range. When they cannot, the device protects itself by slowing down — which is why an overheating laptop feels sluggish rather than breaking outright.
Airflow is the usual failure point on laptops. Vents clog with dust, and soft surfaces such as beds and sofas block the intake underneath. The fan spins faster, gets louder, and still moves less air.
On phones there is no fan at all: the case is the cooling system. Direct sunlight, a thick case and fast charging together can raise the temperature enough to pause charging or dim the screen.
Persistent heat with no obvious workload usually points to software — a runaway process, a stuck sync, or a page playing video in a background tab.