For Congress' part, they did pass laws that make a lot of what Trump is doing now with impoundment of congressional funds, and firing inspectors general explicitly illegal. He's doing it anyway.

But the reason Trump is able to do what he's doing now comes down to the structure of the DOJ being an executive branch he controls. Combined with his immunity from SCOTUS. This means he can argue anything he does isn't a crime, no one will investigate or prosecute him, and he can pardon anyone acting in his direction / direct his DOJ to not prosecute them.

Nothing Biden or Democrats could do about this, because at the end of the day, Republicans decided they deserve this power, and they grabbed it for themselves. It was always there for the taking, they just needed to convince themselves with words and court decisions and speeches that they had the right.