Some activities such as charging or a lot of computational activity (e.g., video), generate heat. Some warmth is to be anticipated.* "Keep your iPhone, the power adapter, and any wireless charger in a well-ventilated area when in use or charging."** Read this article about keeping your iPhone, iPad, or iPod touch within acceptable operating temperatures - If your iPhone or iPad gets too hot or too cold - Apple Support
Consumers want newer device models with larger batteries (= more energy to be lost as heat) and faster charging. This means more heat is released over a shorter period of time, resulting in higher temperatures. If you do not like this, use a lower wattage charger which charges more slowly.
In terms of usage, changes in software such as upgrades may introduce new features which may add to power drain and heat generation. Check Settings > Battery to see if there are apps running that use a lot of power. If there are, decide if you can turn off a service such as Settings > General > Background app refresh when an app is in the background, or Settings > Privacy > Location Services. Read this support article to learn about the battery health feature —> iPhone battery and performance - Apple Support
If you have just updated system software, read posts about indexing in this thread: iPhone 15 overheating - Apple Community
An iOS update may require subsequent reorganization of files and a short term increase in heat and battery drain until the task is done. Wait a few days and see if the issue persists.
If you think there is a problem with a device, have Apple inspect it. Go to this page to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) --> Find Locations
How to make a Genius Bar appointment
1. Go to Genius Bar Reservation and Apple Support Options - Apple
2. Go to "Get hardware help".
3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"
4. Sign in with your Apple ID
*"Your iPhone might get slightly warmer while it charges." - from: How to use your MagSafe Battery Pack - Apple Support
**Section about "Prolonged heat exposure" in this support article --> Important safety information for iPhone - Apple Support
