Most of the Windows tips & instructions online only apply to a traditional dedicated Windows PC and not for Windows BootCamp. Many of those instructions will either break your Windows BootCamp installation and/or your macOS installation. By the time you can press any Windows startup keys you are probably already too late even if those startup keys still work (I'm not sure they do with Win10).
Since Windows did not fully install, I would suggest using BootCamp Assistant to remove the broken Windows installation and start completely over.
Even if you were attempting to troubleshoot a Windows boot issue on a regular dedicated Windows PC, you would find it very difficult to fix things......but dual booting, or on a Mac those difficulties increase exponentially. I used to be very good at fixing Windows boot issues (on a regular PC...not with BootCamp) back in the day (XP and earlier, a bit with Vista & Win7), but the last few times I was unsuccessful due to all the changes in Windows booting that have occurred since then.
Maybe you can find the Windows' installation log to see what may have triggered the failure which may give you more information about what happened. I'm not sure where the Windows' install log is kept, but it would likely be on one of the Window's partitions...likely with the name (or partial name) "install" and a file extension of ".log".
Trying to fix a partially installed Windows installation is even more difficult and may not even be possible. Much easier to just start over.....with BootCamp that means using BootCamp Assistant to remove the broken Windows' installation followed by using BootCamp Assistant to start the Windows installation again.