From all I could see, also from Youtube, I would say this:
In principle any AC/DC converter can be open and modified, to generate any voltage from e.g. 5V to say 12 VDC. The action shown is extremely dangerous and irresponsible.
The Nokia and other chargers have NOT been designed for such modifications. You can kill yourself as well as any device connected to the "modified" charger.
Nokia and other charger designers and makers carefully adjusted the charger to work with a particular expensive battery in a cell phone. Such batteries need the charging voltage and current within specified limits, to achieve a long life. Any unauthorized modification will cause a damage to the battery and/or other device you choose to connect.
If you need a good AC/DC regulated power supply, buy or build one to your specifications. Attempting to modify "a Nokia" or other chargers is possible but irresponsible play with your life (and others), and potentially damaging to connected devices.