Good question.
I just did a Google search and found this.
https://www.washingtonpost.com/politics/2019/11/29/your-impeachment-questions-answered/
Can Trump be subpoenaed in the investigation? Not without a court case going all the way to the Supreme Court.
Why hasn’t Trump’s personal lawyer Rudolph W. Giuliani been subpoenaed? Possibly because he’s Trump’s private attorney and could try to avoid talking by saying everything he knows would be protected by attorney-client privilege
When Trump decided to release the Ukraine military aid after a months-long freeze, what justification was given? Well, Trump has said he was worried about corruption in Ukraine and concern that European countries weren’t doing their part to protect Ukraine from Russian-backed separatists. But the White House didn’t give a specific reason when it released the aid in September after freezing it in early July.
I think the above is a good question. IF it was held up due to corruption and worrying about what contributions others were making then were these addressed and hence Trump then released the aid? What happened to prompt Trump to release the aid?
When the Senate trial begins, will Chief Justice John G. Roberts Jr. be in the position to enforce subpoenas? No. Subpoenas are entirely Congress’s purview. And it will be senators who will decide which witnesses to call. Roberts’s job is to interpret the rules, not set them. As we get closer to a Senate trial
I think this will be very, very interesting. Once the impeachment progresses to the Republican controlled Senate can they turn it instead into a trial of the Bidens or is there something that ensures this trial will be on point to the charges against Trump?