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Most of the 12VDC cell phone chargers use a 34063 chip. Look up the data sheet, and it will have a suggested application circuit which will be very close to what is in your charger. These are easy to modify to other output voltages.
12 vdc would require too much regulator drop in phone for Li-Ion battery charging creating too much heat in phone.
It is switchmode power supply. Simple low cost switcher IC commonly used is TNY264 or TNY268. There will also be one or two opto isolator I.C's, one for voltage feedback, one for current limit feedback.
Output is between 4.4v to 5.0v depending on phone used with.
If charger plug is mini or micro USB connector there will be a pull down resistor on one of two USB data lines to signal circuit in phone that USB power is available.