All basic Arduino functions have been implemented using Arduino C as well as Pure C.
Learning Pure C coding is important as it is used to program various microcontrollers. Getting a hang of pure Embedded C will allow you to write a code for any microcontroller just by going through it's datasheet and understanding it's pin configuration. Arduino Uno has a 32-bit ATMEGA 328p microcontroller on it. To understand the Pure C code given here, it is recommended that you first go through the ATMEGA 328p datasheet to understand the code better.