After you update your iPhone, it must work harder to complete many post-update tasks, mainly indexing your data, in which you can experience an increased battery drain and thermal temperatures. Indexing creates and sorts a searchable database of your data, which powers many features on your iPhone such as Spotlight Search, Search in Photos, Mail Search, Messages Search and many more. Updating to iOS 18 will most definitely experience that as it also has Apple Intelligence features to download. The general rule of thumb for all this is 36-72 hours, but can definitely take longer based on how much data you have.
If your iPhone feels hot, but does not receive a temperature warning message, then it is not considered “overheating” and can continue to be used normally. If it were “overheating” at a dangerous level of use, it would shut itself off until it cooled down.
You can also check out my User Tip: Addressing the Common Increased Battery D… - Apple Community