You're completely wrong on both accounts.
On Windows 10, if you go to Control Panel\Appearance and Personalization\Display, there's a link for scaling. You can choose a lot of values (100% - 125% - 150% etc) or type in your own, e.g. 123%.
Scaling is also per-monitor. I use simultaneously a 27" 5k display, with 175% scaling and my older 1920x1200 24" display, with 100% scaling. Except for when a window is in between them, scaling works fine in either and you can drag a window from one to the other, and when it settles it's scaled correctly.
Even more, I remote desktop into another Windows 10 machine, and I use both monitors, and scaling/dragging through RDP works exactly the same.
While you are correct that you can choose any scaling value you like, and to some extent are correct that it will scale on a per-monitor basis, scaling with displays of significantly differing DPI's is still fundamentally broken in Windows 10 because the scaling value is set by the primary screen when you log on to the device. So if you are away from your monitor when you log in, the scaling value will be set to the laptop's screen. When you connect to your monitor, the elements on the monitor will not be scaled natively, and will be blurry. The opposite will happen if you are connected to the monitor at log in, and then disconnect the monitor to go mobile.
The only way to 'fix' it is to log out again.