Why does my iPhone heat up when I am using the Internet?

1. Because your network signal is fluctuating so very often. Try Wi-Fi.
2. Safari is the best browser for iDevices. Try using it.
3. Using it outdoors. For iPhone 6 and above, the aluminum case shell absorbs heat from direct sunlight. Avoid that.
4. Average battery life of an iPhone is 2 years, usually after that heating issues arise. Try changing one.
5. If you use third party apps that are downloaded anywhere other than App Store. Same applies for tweaks.
6. Use of different amp chargers or power bank may cause heating. Use original accessories. (For iPhone you must use 1 amp chargers only)

These are some of the reason that I know for an iPhone to heat up. Using while charging or playing high-end graphics involved games also may sometimes cause an iPhone to heat up. However, in the extreme heating situations, your iPhone’s screen will turn blue and freeze.