Hello, joly-zjl
Please note that there is Unreal Engine 4 documentation, which is available online.
To use it, please go to Unreal Engine 5.1 Documentation | Unreal Engine 5.1 Documentation
There, you can find a lot of helpful information about Unreal Engine 4. In particular, you can discover functions and variables of a particular class, its parent and derived classes.
Nevertheless, sometimes you may want to look through source code for deeper understanding. A good practice is to look through header files at first, since some useful information can be found there as well.
Hope this helped!
Good luck!