Now that I wonder where I could learn RE? Where do I even start? Got any recommendation of online tutorial or book or something?
A lot of the times, it just happens that someone was the first person that even bothered trying digging into the code. Specially after decompilation became much more accessible for less popular architectures with Ghidra. Give it a try, there's plenty of low hanging fruit! I've submitted another case some time ago.
Also luckily, considering other OS easter eggs, it doesn't seem like there was any obfuscation involved, like "chained xor stored in bitmap resource of badly supported executable format": https://x.com/mswin_bat/status/1504788425525719043