I have a iMac running macOS High Sierra. I ran Boot Camp Assistant on it and was able to switch to Win 10, successfully, without any problem. I downloaded the ISO file for Win 10, and used the USB drive as requested.
However, when I try to switch back to macOS, with the "alt" key held down while restarting Win 10, I never see the screen which shows the different operating systems to choose from. Instead, every restart takes me right back to Windows OS, like it's just a regular Window restart action.