Did you start from the Yuzu documentation? I installed it and walked through their official quick start guide. I didn't type "mario kart free rom" into google or anything.
That's the "legal" way, dumping your own copy of horizon OS, your console's keys, your carts and their keys. Of course it's quicker and easier to just download someone else's keys and dumps, and you're not protecting yourself from Nintendo by using your own stuff, you just get the peace of mind knowing it's your stuff you're using.
It's also very quick if you're already set up with atmosphere & homebrew. Longest part was just waiting for my console's nand to dump.
They aren't (that would make Nintendo's case pretty clear cut), but you can find them fairly easily through the regular channels if you're familiar with piracy.
It's also very quick if you're already set up with atmosphere & homebrew. Longest part was just waiting for my console's nand to dump.