EDIT -- SOLUTION: C:\efi and delete whatever OS you don't need. Also, see if you can delete a boot list from the BIOS. Then, I overwrote whatever I don't need with a new OS (in my case, Manjaro overwriting Ubuntu took care of it).

To clarify, here is what the situation looks like starting from BIOS of two options to bootloaders (not 100% sure what the bootloaders are called:

UEFI(?)               -- Zorin  ------┐
Windows Boot Manager  -- Ubuntu -- Windows

If I boot up Zorin, I have to press a key since Zorin is uninstalled and the bootloader can't find the boot files for Zorin.

If I boot up Windows Boot Loader, I wait 3 seconds for Ubuntu (GRUB I believe?), then wait 3 seconds for Windows (Windows doesn't give me any other OS option anyway, so I don't know why it does that). I have no idea how this came to be.

So now, I want to get rid of both Zorin and Ubuntu and dual boot Manjaro/Windows. How do I approach this situation?