A lot of the customization is in the peripherals. ARM (AFAIK) really only specifies the processor, modern devices have lots of other on-board signals. The inst

A lot of the customization is in the peripherals. ARM (AFAIK) really only specifies the processor, modern devices have lots of other on-board signals. The instruction set really only defines how to run mathematical operations on the data on its bus, what to write to registers, and what instructions should load and store memory to various pages on the memory map. Licensors add peripherals to do things that the instruction set doesn't define, like how to toggle an output pin from low to high. You use the instruction set to set a bit in a particular location in memory, and that turns on an LED attached to that pin - or they make a handy peripheral that lets you write to a word, because setting one bit in a 32-bit memory value is a read-modify-write 3-step operation and many of their devices have thousands of bytes of storage and RAM in an address space with billions of memory locations. They add I2C, SPI, CAN, Ethernet, USB, EEPROM, Flash, SRAM, video, etc. etc. etc. to make useful systems on chip - the processor is only one small part of that equation.