One example is his tax returns. It is a tradition for presidents to voluntarily release them. He chose not to. Then democratic controlled congress fought for several years to have them be released for no real reason except to force him.
Oh you poor delusional soul.
Those tax documents, while evidence of criminal acts, were just a small fraction of the literal 900 bankers boxes of fraud evidence against Trump.
People that understand how our system of government works, understand that Congress has the power of investigation.