Now I tried as follows:
Bios: Secure Boot [Disabled]
PC new start
right mouse click: Start menu - „Terminal (Administrator)“.
bcdedit /set testsigning on
PC new start
OnePlus_setup.exe install
OnePlus_USB_Drivers_Setup.exe install
right mouse click: Start menu - „Terminal (Administrator)“.
bcdedit /set testsigning off
PC new start
Start MsmDownloadTool V4.0 as Administrator - Klick Start - Other - Next - Start
Phone hold Volume Up and Volume Down, plug in USB-cable, release Volume Up and Volume Down
*** Nothing happens ***
Bios: Secure Boot [Enabled]
The files I used I had downloaded from my switched on OnePlus Nord N10 5G phone via USB-cable, becaus I could not find USB-Drivers on the OnePlus homepage.