I am working on a project for a large smart mirror, around 42 inches. To this i will also use a touchscreen for interaction with the screen and the supplier for the touch foil assures that the system is compatible to Linux. Can i assume that i can use the touchscreen on the raspberry pi 3b with any Linux-based system?
You can assume anything you want.
I would not assume that since a device works on one Linux system it will work on all others.
Perhaps the device needs more processing power or RAM than the Pi can supply.
Perhaps the device uses an interface that the Pi does not support.
Perhaps the device relies on closed source drivers which are only available for incompatible CPU architectures.
You will have to test the device yourself.