What they need to show to make a bribery case
Some are making the case that President Trump is guilty of bribery. That means that official acts are conditioned on something of value. For that, the inquiry works on the idea that an investigation into a political opponent is a thing of value. Based on that, the investigation needs testimony to ascertain the following:
That official acts, having a White House meeting and resuming congressionally approved military aid, were conditioned on some sort of investigation.
That the investigation was an investigation into a political opponent. So there are many terms floating around, the company Burisma, Hunter Biden who was on the board of that company and his father Vice President Joe Biden who may be Trump's opponent in the presidential 2020 election.
That the aforementioned points were actually directed by President Trump. After all, if this was something that other officials did without the president's knowledge then he is not the one having done the action that some allege constitutes bribery.
These three points need a lot of collaboration because many of the witnesses have not heard directly from the president. For example, they have talked to Rudy Giuliani, or have been instructed to talk to Rudy Giuliani. It may not yet be clear enough to the American public and those in congress what the president instructed Giuliani to do.
So while there is some case to be made already, it is not enough to satisfy these three points and it is likely that if more higher-ups with more knowledge of the communications the president has had with subordinates on the three points that the facts become clearer.
Why a few witnesses may not answer it all
Specific to your question
Many of the witnesses expose details that were not known previously. But are those details relevant at all for a decision?
That depends and it's not clear before having witnesses testify what they can testify to. So there have been closed door depositions first, these tend to go on much longer and they are really to see what witnesses know.
Then there are the open hearings, which serve a number of points:
The latter point is a separate consequence of public hearings and it may cause others who have decided not to answer subpoenas but have knowledge about the matter to go on the record. That can be for a number of reasons, for example when other witnesses point to them as having coordinated the efforts above.
Fingers are pointing to Rudy Giuliani, but we don't know what he has been instructed to do
In this case, that mostly applies to Rudy Giuliani, who was mentioned by the President Trump in the July 25 phone call with President Zelensky:
Mr. Giuliani is a highly respected man. He was the mayor of New York City, a great mayor, and I would like him to call you. I will ask him to call you along with the Attorney General. Rudy very much knows what’s happening and he is a very capable guy. If you could speak to him that would be great.
That in itself is not putting Giuliani in an uncomfortable position, but combined with the testimony from Ambassador Sondland, it might be. Part of that testimony is stated below as transcribed by rev.com
We worked with Mr. Giuliani because the president directed us to do so. We had no desire to set any conditions. We had no desire to set any conditions on the Ukrainians. Indeed, my own personal view, which I shared repeatedly with others, was that the White House and security assistance should have proceeded without preconditions of any kind.
Searching that transcript for
Giuliani actually leads to 184 hits. The way I understand it, others who were accused of having done something wrong (Amb. Sondland and Amb. Volker), have pointed to Giuliani.
The open question that still remains, at least in my understanding having watched almost all of the testimony, is to what extent Giuliani gave these instructions on his own, or if he was instructed by President Trump to do delegate to others what's stated in my first three points above.