In many apps like safari or youtube the backbutton is at the top of the screen. That makes it basically impossible to use my ipad pro 9.7 comfortably in bed, I would have to sit up and use both hands to access the back button. Is there any way to get a back button at the bottom of the screen?
I'm not aware of any way of doing this within iOS proper (although there may be a way to implement such a feature on a jailbroken device).
However, you could just double tap the home button to then select the previous app from the screen.
Also, using your iPad in landscape (I.e. Sideways) may help with accessing the default 'back' button.
You raise a valid point... with the larger screens of iPad Pros, a worthwhile option would be a system setting that could allow the back button to be relocated.
Unfortunately at the current time, the back button is set at the top of the screen.
This location is controlled by the operating system and so it is outside the ability of a software developer to relocate elsewhere on the screen.
Remember too that many apps allow a swipe action from the left edge of the screen to the right, that performs the same function as tapping the back button.