First, downfall's "source material" as it were, was the interview and memoirs of Traudi Junge, Hitler's personal secretary. She was interviewed about her experiences prior to passing away in 2002. As reflected in the movie, she was personally present with Hitler until the end of his life, and typed his last will and testament.

Hitler did have a famous tirade on 22 April 1945 where he first admitted to witnesses that the war was lost and he informed his top staff of his intent to stay in Berlin till the end and commit suicide.

We do know to a certainty that Jodl was present for that tirade because Jodl later informed Goering's chief of staff who then informed Goering of what Hitler had said. (Goering was in Berchtesgaden at the time). Goering decided that Hitler had become incapacitated and pursuant to a 1941 decree naming him as the successor, sent a telegram to Hitler asking if he should take command of the Third Reich, and stating that if he did not hear back he would assume Hitler was incapacitated and take command. The telegram was intercepted by Bormann who convinced Hitler that Goering was a traitor, and the response sent back was that if Goering did not resign he would be executed as a traitor. Hitler Subsequently drafted new documents naming Doenitz as his successor, believing that the SS and the Air force had betrayed him. Evans, Richard J. (2008). The Third Reich at War.

That said, I'm not aware of any evidence of that scene happening precisely in the way that it occurs in the movie. More detailed descriptions may be out there, I've just not seen them. So we can't say whether it's accurate or the writers taking artistic license with the general tone of what Junge remembered.

Second, the people in the scene are all portraying real people.

Wilhelm Keitel was a Field Marshall and was Chief of the Armed Forces High Command (OKW). He was tried and executed at Nuremburg in 1946, the third highest ranking officer to be executed, behind Doenitz and Goring. (can't verify who they're counting in the source I remember or whether it was merely convictions and not executions)

Alfred Jodl was a General and was Chief of Operations Staff at German High Command (OKW). He was actually the individual who on 6 May 1945 signed the instruments of surrender on behalf of Doenitz. He was tried, convicted and executed at Nuremberg as well.

Hans Krebs was a Field Marshall and appointed Chief of the Army General Staff (effectively the last commander of the German Army under the Nazi Government) on 1 April 1945. He was among those that witnessed Hitler's last will and testament and committed suicide on 2 May 1945.

Wilhelm Burgdorf was General of Infantry the Chief of the Army Personnel Office and Hitler's Chief Adjutant. he was, in some ways, Hitler's Chief enforcer. Or at least he took it upon himself to suppress dissent among the ranks of the Army. Among other things he was one of two general officers that had visited Erwin Rommel on 14 October 1944 in the aftermath of the bombing attempt and given him the choice of suicide or trial as a traitor. Burgdorf committed suicide on 2 May 1945 as well.

Effectively, as of the end of April and the beginning of May 1945, those four men would have been the last high commanders of the German Army/Armed Forces present around Hitler. They would have been natural parties to be present if Hitler were to attempt to direct a last ditch defense of Berlin, or to tell them that the battle was lost and how to manage the last days.

Keitel was Wilhelm Keitel. He was a Field Marshal and was the chief of the armed forces high command. He was the yes men of yes men and generally disliked by other leaders in the German military because he was a lackey who rose to his position through friendship with Hitler and not by skill.

Jodl was Alfred Jodl. He was the chief of the operations staff of the armed forces high command. Jodl signed the instrument of surrender at Reims effectively ending the war however the Russians insisted on their own surrender ceremony as well.

Krebs was Hans Krebs. He was the last chief of staff of the OKH. OKH was part of the army high command that was primarily responsible for the Russian front. Krebs was one of the three people who witnessed and signed Hitler's last will and testament. Krebs attempted to negotiate a surrender with the Russians who were of course in no mood to negotiate nor in a position at that time to need to.

Burgdorf was Wilhelm Burgdorf. He was an army general who at the time of that scene in the movie served as the chief of the army personnel office and as Hitler's chief adjutant.

These men were singled out by Hitler to stay alone with him in the movie scene because they were his high command. Hitler was of course not happy with the way things were going at this point in the movie so he was going to tear in to his commanders and was excusing everyone else.

The movie was based on the books Inside Hitler's Bunker: The Last Days of the Third Reich (1945), by historian Joachim Fest; Until the Final Hour: Hitler's Last Secretary (1947), the memoirs of Traudl Junge, one of Hitler's secretaries (co-written with Melissa Müller); Inside the Third Reich (first published in German in 1969), the memoirs of Albert Speer, one of the highest-ranking Nazi officials to survive both the war and the Nuremberg trials; Hitler's Last Days: An Eye–Witness Account (first English translation 1973), by Gerhard Boldt; Das Notlazarett unter der Reichskanzlei: Ein Arzt erlebt Hitlers Ende in Berlin by Doctor Ernst-Günther Schenck; and Soldat: Reflections of a German Soldier, 1936–1949 (1992), Siegfried Knappe's memoir.

The scenes in the movie are dramatized and inferred from the source material but given the access these authors had, it's probably mostly accurate.