That's false. Quote the constitution which you believe states that. I assure you, it doesn't. In modernity, tradition has it that the AG/DOJ operates independent of the executive branch. The cornerstone of United States jurisprudence is 'no one is above the law', including the president, which means he, like everyone else, is subject to the law. Moreover, States are not part of the DOJ and he is indicted in two states. In order for the justice department to apply the law equally to all citizens, it must operate independently of the executive branch.
Were you alive during Watergate? Clinton? You should know this.
The impeachment process of the constitution is a political procedure. in the DOJ, or state's justice departments, it's a criminal procedure.