Does Windows 10 sleep mode also hibernate system?

So does Windows 10 use a combination of hibernation and sleep instead of «just sleep»?

Yes, that is exactly what Windows is using in your case. It is a mode called Hybrid Sleep. While it is still possible to setup Windows to go into a strictly sleep or a strictly hibernation mode, by default Windows 10 sleep mode is set as Hybrid Sleep on desktop computers if supported by the system. (The command powercfg /a gives you a list of actually supported power states regarding your system.)

From thewindowsclub.com article Difference Between Sleep, Hybrid Sleep, Hibernation in Windows 10/8/7

Hybrid sleep is designed primarily for desktop computers. Hybrid sleep is a combination of sleep and hibernate; it puts any open documents and programs in memory and on your hard disk and then puts your computer into a low-power state so that you can quickly resume your work. That way, if a power failure occurs, Windows can restore your work from your hard disk. When hybrid sleep is turned on, putting your computer into sleep automatically puts your computer into hybrid sleep.

Hybrid sleep is typically turned on by default on desktop computers and off by default on laptops. You will be able to see the settings under Power Options > Advanced settings.