Under U.S. constitutional law -meaning the Constitution itself and the binding judicial precedents and the impeachment precedents (mainly from the failed impeachment of Andrew Johnson)- the president is plenipotent within the executive to do things like:

  - fire principal officers without
    the Senate's advice and consent

  - fire other appointed offices who
    did not require the Senate's
    advice and consent to confirm

  - lay off federal employees in the
    executive branch

  - audit the executive branch's
    agencies

  - set policy for all executive
    branch agencies

The president can also abrogate treaties without the Senate's advice and consent.

Most of the above are not explicitly in the Constitution as such, but are understood to be constitutional law either due to SCOTUS decisions, longstanding and unchallenged practice, or the result of the failed impeachment of Andrew Johnson.

Not only that but also most if not all recent presidents going back decades have done some if not all of the above. That includes Reagan, Bush Sr., Clinton, Bush Jr., Obama, Trump, and Biden.

In other words: there is no innovation here, no judicial controversy. This is all standard fare for any new administration. The only difference is the extent of what Trump is doing in his second term compared to any other recent presidency. The sheer number of EOs, the auditing (which basically hasn't been done recently), and the layoffs (which are rare in DC). And yes, he's goring a lot of oxen -more than other presidents in recent memory-, but they all do that, just not eliciting so much outrage from the opposition.