1- Describe SPI with painting how it works

he asked me about the difference between flash and sram, the diffeence between Avr and Arm architecture, what is the role of the timer in the input capture unit , what is the baud rate that is used with UART and the equation hat is ued in the input capture unit, then he gave me a code problem about a function that takes two numbers and return the substraction of them and what if the two numbers are larger than the integer size what is the solution?