How to use mobile phone socket to supply power to external equipment

The lithium battery of mobile phone is 4.2v (fully charged), while the standard voltage of USB port is 5v. Mobile phone design should consider standby time and other factors, so manufacturers generally don't leave an interface, but if you are an old Nokia, the data lines are in a row, and the second or third one on the left in the middle is for power supply (for those headphones, radios or peripherals). You can refer to it. Now you don't usually bring new ones.