IDA and Ghidra, which they mention at the bottom, along with Binary Ninja which they didn't, have decompilers that will output C source-ish from compiled assembly. Variables and functions without symbols in the binary are just given unique non-meaningful names. There's a lot of AI-powered extensions for them now trying to automate the reverse engineering process of giving them meaningful names, but IMO they are completely worthless and of negative utility - they're either so high level they just reiterate what the assembly is doing but in English, or are wrong and make up explanations for nearly every name that are incorrect and take more effort to correct than they save.