I've been under a rock. Why is DOGE unconstitutional?

I have seen alot of people and left-leaning news sources accusing Elon Musk, Donald Trump, and some DOGE intern named BigBalls of violating the constitution. It's something about investigating spending i think?

My question is, what is happening that's unconstitutional? And how could the Trump Administration alter their actions to make it constitutional? If it cannot be altered, what justification, based in either legality or morality, could be made to defend the action?