If you have a good understanding of c++ then you can go to bitcoin core's repository on github and go through the code. Start from net_processing.cpp as that is where each message comes in and is processed including new blocks, new transactions, etc.
That requires a better understanding of how bitcoin works though which would be better if you start by learning more. The book called Mastering Bitcoin by Andreas Antonopoulos is a very good start https://github.com/bitcoinbook/bitcoinbook