Lt. Col. Alexander Vindman, the Army officer and Ukraine expert at the NSC who testified earlier this week, reported to Hill. Holmes worked in Ukraine and reported to Ambassador Bill Taylor, the top diplomat in Ukraine, who testified last week.
Here are five key takeaways:
Trump's former Russia expert warned Republicans were pushing a 'fictional narrative' on Ukraine
While some Ukrainian politicians did support Hillary Clinton over Trump, U.S. intelligence and a bipartisan Senate inquiry found the primary culprit of election interference was Russia, which engaged in a widespread, invasive and secretive campaign to sway voters in support of Trump.
Trump rebuffed those findings and pressured Ukraine to investigate a debunked theory that Ukraine was really to blame.
"This is a fictional narrative that has been perpetrated and propagated by the Russian security services themselves," Hill testified on Thursday. "The unfortunate truth is that Russia was the foreign power that systematically attacked our democratic institutions in 2016."
Hill testified that this these findings were "beyond dispute," even if some details remain classified.
"I refuse to be part of an effort to legitimize an alternate narrative that the Ukrainian government is a U.S. adversary, and that Ukraine -- not Russia -- attacked us in 2016," Hill added.
She also said Russia's assault on U.S. democracy was ongoing.
"Right now, Russia's security services and their proxies have geared up to repeat their interference in the 2020 election," she said. "We are running out of time to stop them."
The White House countered on Thursday that it was never Trump's suspicion of Ukraine that emboldened Russia, but the impeachment inquiry itself. A White House official told reporters, "Democrats are playing into Russia's hands by trying to undermine the legitimacy of the duly elected president and overturn the voice of American voters."
Hill said Sondland's efforts in Ukraine were a 'domestic political errand'
At one point, Hill described an angry confrontation with Gordon Sondland, the U.S. ambassador to the European Union, who she felt inappropriately interjected himself on Ukraine policy.
"I actually said to him, 'Who put you in charge of Ukraine?'" she testified. "And I'll admit, I was a bit rude. And that's when he told me, 'the president,' which shut me up."
She said that what angered her was that he wasn't coordinating with the experts on Ukraine, Russia and Europe at the National Security Council. But when she saw him testify on Wednesday, she said it hit her that Sondland was operating with the explicit permission of the president's top advisers.
Sondland testified on Wednesday to pressing the Ukrainian government into announcing the investigations that Trump wanted and, at one point, told them nearly $400 million in military aid was on the line. He said he was working with Trump's personal lawyer Rudy Giuliani, and that "everyone was in the loop," including acting White House chief of staff Mick Mulvaney, Energy Secretary Rick Perry and Secretary of State Mike Pompeo.
"He was absolutely right because he was being involved in a domestic political errand, and we were being involved in national security foreign policy and those two things had just diverged," Hill testified on Thursday.
Holmes described, in vivid detail, the Trump-Sondland phone call in Kyiv
Holmes was the State Department staffer who took notes on meetings between U.S. and Ukrainian officials, including Sondland, during meetings in Kyiv.
Holmes described going to lunch with Sondland and other staffers afterward. Sitting on a restaurant terrace, across the table from Sondland, Holmes said he could hear from Sondland's cellphone, Trump's voice shouting -- his "very distinctive personality."
Trump asked Sondland whether he was in Ukraine, Holmes told lawmakers on Thursday.
Sondland "said 'yes,' I'm here in Ukraine and then Ambassador Sondland said 'he loves your ass. He'll do anything you want. He's going to do the investigation,'" Holmes testified.
Holmes says Energy Secretary Perry excluded embassy personnel in talks with Ukraine
Perry, who announced his resignation last month as energy secretary, has ignored a congressional subpoena for documents on his role in Ukraine. The Energy Department says Perry was only aware of an administration effort led, in part by Sondland, to press Ukraine on anti-corruption efforts.
But Holmes provided details on Perry's involvement in Ukraine, including that his staff "excluded" embassy personnel from listening in on meetings with Ukrainian officials. Perry, along with diplomat Kurt Volker and Sondland, were picked by Trump to work with his personal lawyer, Giuliani, on Ukraine issues, according to testimony by Volker and Sondland.
Holmes also testified that Perry, a former Texas governor with deep contacts in the energy sector, passed along a list to Ukraine President Volodymyr Zelenskiy of "people he trusts" who could help the country with energy reform.
The Associated Press reported earlier this fall that Perry tried to put a previous campaign donor on the board of a Ukrainian gas company. Perry told reporters, "we get asked for our recommendations about people who are experts in areas" and that his recommendation were "folks who have expertise in particular areas."
Holmes told lawmakers on Thursday that after Perry passed along his recommendations, he met with "key Ukrainian energy-sector contacts" without diplomatic staff present.
"Embassy personnel were excluded from these later meetings by Secretary Perry's staff," Holmes testified.
Perry's office did not immediately respond to a request for comment.
Bolton described as 'pained,' resigned to Giuliani
Hill worked for John Bolton, the conservative defense hawk who abruptly resigned in September as Trump's national security adviser. And throughout her testimony, Hill made clear that Bolton wasn't in line with what Sondland and Giuliani were trying to do.
It was Bolton who told Hill to report Sondland and Giuliani's pressure campaign on Ukraine to legal counsel, she said. And he specifically told Hill to relay the message that he opposed, "whatever drug deal" the Sondland and acting chief of staff Mick Mulvaney were cooking up.
In a statement released to reporters on Thursday, Mulvaney’s lawyer said that Hill's testimony is "riddled with speculation and guesses about any role that Mr. Mulvaney played with anything related to Ukraine."
At one point, she testified that she remembered talking to Bolton about Giuliani's false and public "smear campaign" against then-Ambassador to Ukraine Marie Yovanovitch. Hill said she asked Bolton if there was anything they could do.
Bolton, "basically indicated with body language that there was nothing much that we could do about it," Hill testified on Thursday. "And he then, in the course of that discussion, he said, that Rudy Giuliani was a hand grenade that was going to blow everybody up."
ABC News' Stephanie Ebbs and Jordyn Phelps contributed to this report.