Part 1

What you are experiencing when not finding any viable material is a sort of confusion that results from the imprecise and superficial use of certain terms such as Holocaust in popular culture: In addition or rather as a prelude to the Holocaust – the state-sponsored mass murder of Jews and those termed as "gypsies" such as Roma, Sinti, and others – the Nazis implemented a state-sponsored program of mass murder targeting people they regarded as disabled due to physical disabilities or mental illness and/or disasbility.

While the Nazis termed this their "Euthanasia program", scholarship most commonly refers to it as the "T4 program" – a name coined in the post-war years referencing the street address of the Chancery of the Führer – Tiergartenstraße 4 – where the program was initiated. Between the years 1939 and 1945 the T4 program resulted in the mass murder of about 300.000 disabled and mentally ill people in Germany, Czechoslovakia, and Poland with most victims being killed during the "official" program running between the fall of 1939 and the summer of 1941 when wide-spread public protests lead to the announcement of the end of the "official" program. The killing of handicapped and mentally ill people however would continue pretty much until the end of the war in a decentralized fashion.

The backdrop for this mass murder program was the Nazis' believe in the assumptions "racial hygiene", meaning the then world wide believed and wide-spread belief that by somehow eliminating people with disabilities, mental illnesses and other hereditary diseases from the gene pool of a population, this would then lead to those diseases and handicapps disappearing. This belief had its horrible fallout in various contexts – the Nordic countries and the US would forcibly sterilize people f.ex. –but none as horrific as in Nazi Germany.

/u/kieslowskifan has written about eugenics and its history and how it intertwined with what became known as the T4-program and its aftermath before in this answer where they mention that in the 1930s Nazi Germany instituted a program of what they referred to as "racial hygiene" that enabled health courts and other state institutions to mandate the sterilization of people deemed not worthy of procreation – a program that exorbitantly affected the poor and socially marginalized, including f.ex. alcoholics and people who suffered from other, what are now known as, social diseases often brought on by poor and mentally taxing living conditions.

Together with this campaign of racial hygiene, the Third Reich ramped up its propaganda towards portraying people in institutions as "useless eaters" that only cost society money without contributing to it. Images such as this one that portrays a healthy German carrying these people on his shoulders paired with a text that highlights how much it cost to feed and care for someone in an institutions are examples of this kind of propaganda.

There had long been talk among the Nazi leadership about how this "problem" was to be addressed but the outbreak of the war proved a decisive moment in radicalization of policy. Because the leadership perceived it as the start of the ultimate struggle for "racial" supremacy and the final conflict between various races, especially the "Aryans" and Jews, they felt empowered to finally institute measures that would enable to shape German society and demographics in a way that would give them, according to their warped world-view, a leg up in this conflict. Among these measures was to start a targeted program of killing the "useless eaters" in German institutions, hospitals, and asylums for they were perceived as unable to contribute to the winning of the conflict.

After a trial case where a Saxon man petitioned Hitler to be able to euthanize his disabled son, who was killed in 1939, Hitler in October 1939 wrote a memorandum backdated to September 1 to the head of his chancellery, Philipp Bouhler, and his personal doctor, Karl Brandt, that assigned them the task of "empowering certain medical doctors to euthanize people who count as incurably ill under all considerations and after critically reviewing the state of their illness ["dass nach menschlichem Ermessen unheilbar Kranken bei kritischer Beurteilung ihres Krankheitszustandes der Gnadentod gewährt werden kann"].

What followed was the build-up and institutionalization of what has become known in historiography as the T4-Action (T4 after the address of its headquarters, Tiergartenstraße 4) but what the Nazis called the "euthanasia program". Bouhler, Brandt, and Hans Lammers, head of the Reich Chancellery, build a program that consisted of founding a variety of companies intended to camouflage their program such as Gemeinnützige Krankentransport GmbH (Gekrat) responsible for the transport of mentally and physically disabled or the Reichsarbeitsgemeinschaft (RAG) „Heil- und Pflegeanstalten“, responsible for generating lists of those to be killed. With the help of Reich Health Leader, Leonard Conti, they ordered all asylums and hospitals in Germany to generate lists of all patients, who suffered from diseases such as Schizophrenia, Epilepsy, Encephalitis, Chorea Huntington, "feeble-mindedness" or any other neurological condition that prevented them from carrying out "mechanical labor"; people who had been institutionalized for more than five years; mentally ill criminal offenders; and people in institutions who weren't German or of "related blood".

Based on the information collected by these institutions, a panel of three medical doctors designated from a pool of about 40 designated experts was to determine what would happen to these people. In practice, these so-called experts often knowing very little about the patients themselves since all they had was one sheet filled in by the institution that housed the patients in questions, often decided live and death of these patients on very flimsy criteria. Once their fate had been decided by these "experts", the patients in question would under the pretense of bringing them to another institution better equipped to handle them be transported to one of six central institutions that served as killing centers, namely the hospital institutions in Grafeneck, Brandenburg, Hartheim, Sonnenstein, Bernburg, and Hadamar.

Assembled for this very purpose by the Reich Security Main Office and the T4 office, a group of 300 people or so worked in these institutions to ensure the working and proper execution of the killing program. They build gas chambers following a recommendation of the Criminal Technical Institute of the German Police and installed crematoria ovens in these institutions. This was where mass killing by gas chambers was first pioneered by the Nazis, especially in terms of the logistical and methodological considerations as how this was handled would later starkly influence this process in the killing centers in the East (a subject I'll return to), including the famous disguising a gas chamber as showers.

After the patients had been murdered, a special Standesamt (the German state office responsible for recording births, deaths and marriages) that had been attached to these institutions would issue false death certificates stating that the deported patients in questions had died of heart attacks, their lungs giving out and similar causes of death and send them out to their relatives and family.

This centralized T4 killing program was in operation in Germany from 1939 to 1941. During that time more than 70.000 people were killed in these six centers. During this time, the German public gained more and more a sense of what was occurring. People who lived near the centers noticed the distinct smell of human cremation coupled with the strong smoke from the facilities. In Hartheim, the cremated ashes of patients were simple thrown into the nearby Danube, which made it extremely noticeable to the locals. Furthermore, the death certificates sent out to relatives were often faulty, impossible in what they detailed as the cause of death or were sent twice. Lothar Kreyssig was a German judge whose main job it was to be the legal guardian of mentally disabled people unfit to partake in legal exchanges. When he noticed that some of his wards were being killed, the reported it duly to the ministry of justice. The ministry told him that this was occurring on orders of the Chancellery of the Führer and so Kreyssig did what made sense to him: He went to the police to report Philipp Bouhler for murder. While the investigation was killed and Kreyssig retired all these occurrences spread knowledge of what was going on in the German populace.