Just 30 minutes before writing this question I installed Ubuntu on an external 500 GB HDD I had lying around (so I could mess around in Linux when I'm in the school computer lab), and I did so by booting from a 32 GB USB thumb drive which I put the Ubuntu iso on with Rufus.
I opened the installer and triple checked that I wasn't installing stuff on my main Windows drive, so I clicked continue and after some time the installation finished.
I was still scared that I might've fucked up something and somehow installed stuff on my main drive, so I booted up the PC after removing the thumb drive, and... boom. Black screen with a GRUB command line(?). I started panicking, so I shut down again and opened the boot menu, and there it was: under my main drive, there was a UEFI "ubuntu" boot option, and under it was Windows Boot Manager. Windows boots perfectly fine, and all my files are here, but why the hell did it install GRUB on my main drive when I clearly selected the external HDD as the install destination???
I tried moving WBM up in the boot order settings, but it won't let me for some reason. How can I uninstall GRUB so I don't have to go through this mess every time I want to use my PC??
P.S. sorry for making such a long post for what seems to be a pretty easy problem to solve, I just wanted to give full context to anyone reading.