is the source code available in its entirety ?

Hello, (Disclosure i’ve only used c++ opengl or unity until now)
so i’m thinking of starting to use unreal engine because some people told me that the source code for the engine is available and can be modified, which means i would have a huge advantage when it comes to control and flexibility over using something like unity, my question is: is that correct ? can i access the source code, change it ? use it however i want ? if so how ?

Yes, the source code is available, however that does not mean that it is open-source. You must agree to the licensing terms, and one of the main things is that you can’t redistribute the source code or use it outside of the engine.

All source code for the engine is available, including all 3rd party libraries that were used (except for FBX)