According to Politico mainly intelligence officers have been questioned so far in this investigation:
Durham and Barr have been focusing primarily on the intelligence community—reportedly seeking interviews with the CIA analysts who drew conclusions about Putin’s motivations in 2016—and have not requested interviews with any of the senior FBI or DOJ employees who were directly involved in the opening of the Russia investigation in 2016, according to people familiar with the matter.
The Mueller connection being that their info influenced the FBI:
The F.B.I. did not use information from the C.I.A. in opening the Russia investigation, former American officials said. But agents’ views on Russia’s election interference operation crystallized by mid-August, after the C.I.A. director at the time, John O. Brennan, shared intelligence with Mr. Comey about it.
The C.I.A. did contribute heavily to the intelligence community’s assessment in early 2017 that Russia interfered in the 2016 election and tried to tip it in Mr. Trump’s favor, and law enforcement officials later used those findings to bolster their application for a wiretap on a Trump campaign adviser, Carter Page. [...]
In interviewing more than two dozen former and current F.B.I. and intelligence officials, Mr. Durham’s investigators have asked about any anti-Trump bias among officials who worked on the Russia investigation and about one aspect of the investigation that was at the heart of highly contentious allegations that they abused their powers: the secret application seeking a court order for a wiretap on Mr. Page. [...]
Mr. Durham has also asked whether C.I.A. officials might have somehow tricked the F.B.I. into opening the Russia investigation. [...]
Mr. Durham also has yet to question many of the former F.B.I. officials involved in opening the Russia investigation.
(Mueller was later appointed "to oversee the previously-confirmed FBI investigation of Russian government efforts to influence the 2016 presidential election and related matters".)
And while the NYT did not quite call the Obama-era intelligence leadership (Brennan and Clapper) as being directly targeted by Durham (only saying that he probably wants to interview them), Trump was less roundabout:
Mr. Durham has indicated he wants to interview former officials who ran the C.I.A. in 2016 but has yet to question either Mr. Brennan or James R. Clapper Jr., the former director of national intelligence. Mr. Trump has repeatedly attacked them as part of a vast conspiracy by the so-called deep state to stop him from winning the presidency.
The NYT also points out that Durham has led some investigations into the Bush-era CIA, so probably that was a reason why he was selected for this job.