The real history is all out there, why did they feel the need to make things up? Oh that's right, it's HBO.
There are several excellent Chernobyl documentaries out there, including one made by the Ukrainians, which comport very well with the story recounted in the mini series. Some of these are available on Amazon Prime or Netflix. The Russian ones are in Russian with English subtitles. Dyatlov comes off as an overbearing, arrogant ass who did not follow established protocols for the test in all of them. The Ukrainian one was made in the still operating reactor control room of Chernobyl Reactor #2 in 2005 and used actors who looked exactly like the original people. It was uncanny.
There were fictionalized characters in the mini-series such as the female physicist who did the interviews of the irradiated workers. If you had watched the after program “podcast” discussions of the producers, writers, and directors, you’d know she was a composite character of all the scientists who had investigated and did the interviews to keep the story simpler for filming. They discussed this completely and the reasons for it. . . Otherwise they would have expended too much screen time in portraying meetings and discussions. The KGB events that occurred to her did indeed occur to various investigators and the roadblocks put up by the state and KGB to uncovering the facts of the RBMK reactor that had been classified by the state had also been run into by the various team members when trying to discover why it happened.