Thank you very much for your answers.
I’m still a bit unsure about the project properties and the rendering menu templates.
As I understand from your answer, whatever project properties you choose allow all renderings? Does that mean that the only thing the properties basically do, is add = sign for renderings that (partly) match the properties? In other words, they give you a framework/guidance of what you should do?
As for the pixel ratio, I started trying out some more things. Now I notice the following. If you go to the video properties of the file you can select the pixel ratio of the video itself and the video gets stretched according to what you selected. If you go the project properties the total image with black borders is cut of accordingly, while not changing the video image. So as I understand these aspect changes are only really relevant for the preview, because when you go to the rendering the video is rendered in the default (= dependent on the video input) pixel ratio, right?