I haven't used hyprland. I can answer your questions for XMonad, assuming you're using a typical standard layout.
> If you created a window right now, where will it go?
The new window becomes the focused window. It's inserted into the master position. Existing windows shift down the (conceptual) stack.
> Does it use your focused window?
It uses the same screen space, yes.
> Which window will it take its space from?
All of the other visible windows. It recomputes the tiles so that all tiles except the master become smaller, to make room for the new one.
> Your mouse position?
By default, mouse position is ignored. XMonad is keyboard-centric by design. You can set a mouse-follow configuration variable if you want. I've never tried it.
> If your WM supports mixed floating & tiling, how does it go when you flip a window between them?
It recomputes the tiles in much the same way as above. It's as though you deleted the window from the tiling and it becomes floating. And vice versa. It's a very consistent model.
I find it very natural and predictable. As far as "cognitive load" goes, that seems like an exaggeration, but again I haven't used hyprland.
If by "hand control" you mean using the mouse, that's definitely not needed for window management. In fact by default, XMonad doesn't even support resizing tiles using the mouse, and I've never tried to enable that. I do commonly use the mouse for switching focus, usually because I'm navigating to some location in another window anyway, in which case focus moves automatically.