Q: iphone 6s gets hot when using internet by wifi

Hi immo85,

I understand that your iPhone's getting warm when using it and connected to Wi-Fi. This can be expected, as your iPhone's exterior is a part of the device's cooling system. As the components that drive the device generate heat, the exterior is used to cool the internal components by moving heat to the outside of the iPhone.

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures
https://support.apple.com/en-us/HT201678

Cheers!