I don't know how they did it, but a company named Topf & Söhne ("Topf & Sons") built the cremation ovens and even filed a patent, which got rejected because "muh optics". Topf&Söhne was very proud of their technical prowess. They also built piping to use the waste heat from the ovens for tele-heating the gas-chambers. The wikipedia-articles are fairly superficial. I tried to research that, but pertinent documents are not on the internet. A job for Germans who live close to the paper archives.