That is partly true: invisible tasks such as indexing your media library or updating system databases really do load the processor more than you would think. But there is a scenario people mention less often — heat at the moment when the phone is thinking rather than working.
Take a simple example: after a major iOS update the phone stays warm for a couple of hours even with the screen off. The platform is rebuilding Spotlight search indexes, processing photos for face recognition and reassembling Siri's predictive chains. Peak load falls in the two to four hours after installation, but some background operations can stretch over a whole day.
The second quiet culprit is automatic backup. If iCloud is set to sync over mobile data while you are on the move, the modem works in constant tower-searching mode. Combined with active data transfer, that produces steady heat around the logic board. The simple fix is to switch backups to Wi-Fi only and see whether the warmth remains.
There is a hardware side worth remembering too. After two or three years of heavy use the thermal paste under the A-series chip dries out and heat transfer gets worse. You will notice it in everyday use rather than in benchmarks: the phone warms up during a video call, in simple games or while browsing photos. A simple test helps — five minutes of 4K video recording, then compare how warm the body feels against the usual.
Owners of models with MagSafe often notice heat right around the wireless charging coil. This happens outside charging as well: if the case is third-party or simply thick, heat from the Wi-Fi and Bluetooth chips dissipates poorly and concentrates in the upper third of the body.
Diagnostics are easiest to start from Settings → Battery: look at which app consumed power over the past 24 hours and how long the screen was on. If nothing among the leaders looks suspicious and the body is still consistently warm, the next step is to check the thermal sensors and the battery's condition.
A warm body with no visible load is the language the system uses to report background processes, worn components or a charging pattern. Each cause has its own handwriting: when it appears, where the heat sits, whether it followed an update. Watch the device closely a few times and the picture becomes readable.