Let's see. Under Clinton, the inquiry was held in private under special council Ken Starr. Likewise, the Russia probe, was conducted privately by special council Mueller.  On the subject of obstruction of justice, Mueller said he had been prohibited by Justice Department policy from charging the president with a crime and that any potential wrongdoing by a president must be addressed by a "process other than the criminal justice system."

When Trump released the partial transcript of the phone call with the president of the Ukraine, the house began an inquiry, which is being conducted as prescribed by Article II. Nothing in that document details any blueprint for the proceeding, only that it must occur.

We have had exactly three instances of this in US history, as Nixon resigned before he was impeached.

Interestingly, just because Mueller was forbidden by Justice Department policy from charging the president with a crime, this does not mean that it is a law. The very concept of this was discussed today in the Second District Court of appeals in Trump v. Vance, and might possibly be addressed within the decision on that case, which pertains to the release of Trump's tax returns.
 
The hearing was broadcast live and can be heard HERE:

https://www.c-span.org/video/?465172-1/c...audio-only

Anything else you need clarification on, please don't hesitate to ask.

---