How to 'separate' a vertical oriented monitor?

Unfortunately the built in "Window-Snapping" as Microsoft call it doesn't let you do quite what you are looking for. The closest you can get is snapping to quadrants, to do this you snap the window to the left or right as before but without releasing the Windows key follow the left or right press with an up or down as well. The window will then be "snapped" to that quarter of the screen.

To get the full width you'd then have to use the mouse to stretch the window across the screen which is less than ideal.

The best solution I've found so far is to use DisplayFusion which includes the functionality to setup hot-keys for doing this and I believe this functionality included in the free version as well.

Windows does not do that. The closest that it can do is to use Snap Assist to snap a window to a quarter of the screen by dragging it to the respective corner of the monitor.

You need a more sophisticated windows manager for that. For a list of third-party products see the article
10 Tools To Intelligently Split Your Monitor’s Screen.

Does anyone know how this can be changed?

As shown by the other answers, you cannot change that behaviour by onboard mechanisms of Win10.

You can take a look at AutoHotkey (AHK) though, it has this or that user-provided script to do what you want.