Wireless charging uses magnets to transfer power from the coil that is used to transmit in the charging device to the coil that is in the smartphone. Nowadays you can even buy high capacity wireless charger for more efficiency.
To allow this energy transfer wirelessly it is necessary to perform several "conditionings" on the signal that powers it. They can result in energy loss, which can cause the production of heat.
The blocks on the left are connected via the help of an electric cable and an outlet of electricity (it may be a power outlet that is located on the wall or a small battery that can be carried around). Each block can lose energy through heat and the largest loss takes place in the receiver as well as transmitting coils. On top of that, it is possible to also lose heat produced by the parts of the phone, such as the charger itself.
In the end, wireless charging is going to be a feature that is an extra feature for most consumer electronic devices, such as smartphones. To be a hit with customers, it should be financially viable for companies that have charging stations in places for business such as restaurants and offices and for people who buy separate furniture items and chargers with wireless chargers.