What books or other online resources cover programming in C/C++ for serial communications such as USART/UART.

There is a program I am studying that sends the string "hello world" over a USB from a USB-Serial adapter cable and an AVR ATmega 328p MCU.

Platform agnostic, I want to focus in general, how does serial communications work and how do programs in C (or C++) work when it comes to sending a string of characters using USART or UART.

Update: I found "Timer.1 - Using a timer synchronously" on Boost.org. This is great. Does anyone know if "Boost.asio C++ network programming cookbook" is a good book that starts with basics?

Another update: I found an online tutorial for Embedded C programming for Arduino/ATmega328p about USART.

https://www.arnabkumardas.com/arduino-tutoria … /usart-concept/

I have also wanted to ask how should I go about learning more about the hello world over RS232 to USB (Serial to USB) cable? I was not on track when I first thought to ask about serial communications programming -- that is not what I needed.