Paired with Ghidra having a binary, being able to do a memory dump of a live running program, and being able to use wireshark to dump traffic over network/bluetooth/usb is VERY helpful if you don't have the source code.

You use decompilation tools and hope they left debug symbols in and it turns it into somewhat human-readable language which is often enough. Even when you don't binaries use libraries which are known or at some point hit documented interfaces so things can be reasoned about.