Hello,
Serial communications:
https://learn.sparkfun.com/tutorials/serial-communication/all
Connecting Arduino to Processing:
https://learn.sparkfun.com/tutorials/connecting-arduino-to-processing/all
Some good stuff here:
https://itp.nyu.edu/physcomp/ You will have to search through categories.
Processing Serial Library:
https://processing.org/reference/libraries/serial/index.html
This examples and documentation are simple but could use some updates and corrections for clarity.
As I said before do a search and find what best suits you.
There are plenty of topics in this forum as well.
Serial communications is simple but can be daunting at first… once you get over the initial learning curves it gets easier.
:)