Evidence has strengths and weaknesses. Eyewitnesses are, due to memory and recall issues, often unreliable, so they are weak when it comes to details. But they provide the strongest narrative, chronological evidence. Forensic evidence is generally very reliable, but it is a snapshot that provides little narrative. Photographs are the same. Circumstantial evidence can be very weak, or very strong.
At Nuremberg (though the majority of Holocaust related trials came later, or they were not at Nuremberg) the circumstantial evidence was very strong. Numerous reports had been coming from Nazi occupied Europe, thought out the war, of mass arrests, seizure of property, transportation and imprisonment of Jews. That is also evidence of opportunity, as it gave the Nazis the opportunity to kill. From 1941, there was a steady stream of reports of mass murder, which by 1944, was in the millions. In 1945, the Allies, east and west, liberated only a few hundred thousand Jews and many countries reported that few returned home. When questioned, Nazis either admitted to mass killings, denied knowledge of or responsibility for mass killings or they claimed they believed Jews were being resettled, but they could not give any details about where and when.
If someone declares open hostility to another, and they then kidnap that person and that person disappears, that is circumstantial evidence of murder. When that someone admits to murder, a witness comes forward to say they saw the murder, and some human remains are found, that is enough corroborating evidence to convict. Except, when it comes to Jews being murdered by Nazis, for some reason, according to so-called revisionists.