The ORIGINAL source code is copyrighted, but decompiling does not give you the original source code. Decomp tools give you generic variable names like unsigned_int_4 and then it’s up to you to decipher what the purpose of the variable even is and give it a relevant name. So it’s virtually impossible you’ll get a character-for-character match to the original.

Also, decomps have different levels of accuracy. You may get something that is a perfect behavioral match, even though there’s differences in the instructions being run. You may get an instruction-perfect match but not a byte-perfect match between the binaries.

IANAL but this is what I’ve learned from following decomp projects on YouTube

Yes, reverse engineering is totally legal. The big asterisk here is that you can't distribute any assets the company owns, so you need the original game files regardless to play the decompiled version.

"Legal" is a very strong word here.

A better statement would be that there hasn't really been a major court case regarding decompiling source for hobby use and most cases where it is used for profit or piracy are no nos for existing reasons. But for education purposes or to develop and interface, it is a grey area. In large part because said education makes you toxic as hell in industry and said interfaces are usually for things that violate the license agreement and TOS because the software didn't provide an interface for a reason.

And then you run into cases where the act of bypassing protections to get the binaries in the first place put you in further hot water. Sort of like how the (way over simplified) argument that you are legally allowed to have a backup of your video games but the act of making said backup gets REALLY sketchy in a lot of cases.

I would probably phrase it more as "decompiling source code is not illegal. How you use it might be"

Its also always worth remembering that law comes from precedent. And it is rarely in anyone's interest for stuff like this to go to trial. So there will be a few landmark cases that cover VERY specific use cases and a LOT of cases of big lawyers saying "Do you really wanna fuck with us?" and getting a C&D and a settlement.