They did it throughout the war and not merely at the start? So weird that there's photographic and video evidence of people being gassed in buildings then.

Edit - Here's evidence that the Germans used chambers (often called "showers") from 1941 onwards:

https://www.theholocaustexplained.org/the-camps/types-of-camps/ https://encyclopedia.ushmm.org/content/en/article/gassing-operations

While I'm sure vans were still used for their mobility/convenience and in cases where not many people were to be executed, Germany created extermination camps specifically to kill people (in more ways than just gassing) and those locations contained constructed buildings meant for execution with gas, not vans.

Even a few concentration camps had their own gas chambers, which were not vans, that were used to execute people that could no longer do forced labour.