Attempted dual-boot goes straight to Windows 10

Thanks to @PonJar:

It turns out my problem was mainly not booting the USB stick in UEFI mode. This caused the Ubuntu installer not to recognize Windows and also caused it to install in BIOS mode, meaning the computer will never use it when turning on normally!

I solved the problem by following this guide on installing in UEFI mode. Note that under step 6, it says that when using manual partitioning you will need to set the mount point on the EFI partition to /boot/efi -- I wasn't able to do this but still installed fine.

Another thing to note, for those like me without much experience with partitioning: make sure the "Device for boot loader installation" is set to the entire drive you'll be using as opposed to just the Ubuntu partition. It should be set properly by default -- I had changed it and I believe this was also part of the problem.