The below YouTube video gives a good introduction to the Bitcoin source code, and the principles behind it. As Bitcoin is decentralised, and there is no “official” organisation behind it, it can be a bit difficult to find the “authoritative” sources of information about it. I have a page about Bitcoin and cryptocurrencies, and in the section “Bitcoin Coding and Development”, you’ll find some useful links to get you started on understanding Bitcoin better.
If you want to view the Bitcoin Source code, which is written in C++, then you need to visit its page on GitHub below:
Then go to the src folder:
You can open the .cpp files to see the actual code. Some of the commonly modified code resides in the rpc and wallet folders, so you can go down a level and look at some of the .cpp files in them to start familiarising yourself with the source code.
To help get into Bitcoin development, you can also join a Bitcoin Core Pull Request Review meeting, which currently take place over IRC on Wednesdays.