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#160
It depends upon how the BIOS/UEFI is set up. Your BIOS/UEFI can be set up to boot from the installed hard drive/SSD first. In that case, you press a key to get into a boot menu which allows you to override the default setting and boot from the USB or DVD instead. In that case, the override is for that boot only and when Windows setup reboots the computer, it will follow the order set in BIOS/UEFI and boot from the installed hard drive/SSD first and then continue the setup process from there.
Or your BIOS/UEFI can be set to always boot from a USB/DVD first - if one is present. In that case, when Windows setup restarts the computer during setup, the BIOS/UEFI boots from the still inserted USB flash drive or DVD and the setup program starts again from the very beginning because the Windows 10 USB flash drive or DVD is not modified during the setup process. In that case the solution is to remove the USB flash drive or DVD and force the computer to restart and boot from the installed hard drive or SSD and Windows setup will continue from there.