Ah, so you took the first path. Nice.
He may owe back taxes. Because we can see his released taxes we can see how he filed and what taxes he paid. But every news story I've seen is careful to say he 'may owe taxes' and the s-corporation 'may have been misused'. Nobody is saying anything definitive.
Also, this isn't really new. It was reported by the WSJ back in 2019, but Trump couldn't really use it against him without bringing attention to his own taxes. https://www.wsj.com/articles/joe-biden-used-tax-code-loophole-obama-tried-to-plug-11562779300
If he cheated on his taxes, I hope he is at a minimum fined. Whatever the law calls for should happen.