4 usb pins and programming what part of the section in electronics?

Question

Electronics

Brothers, please tell me a book or an article about the programming process and why more than 4 pins were required before, but now 4 will do to program the MK, and how the process itself works.

Answer 1

Microcontrollers are becoming more complex - they are overgrown with built-in hardware implementations of interfaces, the division of flash memory into a bootloader and a main program.
Now the microcontroller flashes itself (bootloader) via a serial interface, earlier almost direct recording of the built-in memory was used during programming - via a parallel interface - hence a bunch of wires.