Good point, interested in the answer myself.
The answer is the Congress.
If the Congress deems acts perpetrated by the President is a high crime or misdemeanors, they can impeach him.
The corrupt Democrats "deemed" Trump committed several of these acts, and impeached him twice.
But to avoid EXACTLY the type of purely political impeachments the democrats tried, the brilliant authors of the Constitution said the trial would be held in the Senate, and 2 thirds of them would have to agree to convict.
Obviously they did not convict.
BUT, if a President did commit an act heinous enough, the thinking would be that even members of his own party would agree to convict.
IMO Biden has several times reached that threshold. Maybe more.
Certainly his refusal to secure our border from foreign agents and untold MILLIONS of illegal immigrants warrant impeachment.
Not to mention the treachery he committed in the manner in which we left leaving Afghanistan.