Easy.
18 USC 912 makes it a crime to impersonate a Federal officer. When Biden's handlers took charge, they ultimately assumed his power, which is a violation of federal law.
18 USC 371 makes it a crime to knowingly conspire and defraud the United States. By knowingly misrepresenting Biden's state of health to the public in an attempt to have him elected again, so they could continue the ruse, they were committing a high form of fraud. One could argue that by this action they were also misleading the various arms of government into believing he was of sound mind and body, which is fraud, again.
18 USC 2385 makes it a crime to assume the powers of the president, IE subverting the Constitutional order, which these staffers and advisers did, similar to 18 USC 912. It also violated Article II and the Succession Clause of the 25th Amendment.