https://stellafane.org/tm/newt-web/newt-web.html
Poider you may find this calculator useful for visualizing (ray trace tab) what needs to be to done to make a particular telescope achieve focus.
It will also give you an idea of the performance of different configurations under the performance tab. In particular this will show you if you can fully illuminate a DSLR sensor or not.
Put some coffee on, its a lot of fun.
You will see that raising the mirror, or lowering the focusing tube radially or axially has the same effect of pushing the image plane out of the telescope. That is what is happening when people shorten their OTA length. Of course there are caveats with regards to secondary size etc. but you'll get the idea.