I have a Fedora installation on one of my SSDs with GRUB installed to an EFI boot partition. I installed Windows 10 using a live installation image to the other SSD, and now I cannot find Disk1 (the SSD w/ Fedora and GRUB) in BIOS, and hence cannot boot into it. What happened, and how can I restore my ability to boot this disk?
Windows disk manager, displaying that the EFI boot partition still exists
You have UEFI, not BIOS! Although the (new) UEFI performs the same function as the (old) BIOS did, UEFI is not BIOS, and the way it boots one or more OSes is entirely different.
According to your Windows Disk Manager screenshot there's only one ESP, the EFI System partition (correct) so, if Windows boots, everything is as it should be.
You won't need to change drive boot order. Actually this concept is mostly applicable to BIOS/MBR systems although in an UEFI system with multiple drives we may need to assure the drive containing the ESP is at the top of the list. Again, if Windows boots then the correct drive - Disk 1 - is being used, even though Windows is installed in a different drive, Disk 0.
Regarding your specific case, because you installed Windows after Fedora, it changed the bootloader order (different from drive order) to its own (Windows bootloader manager). The Windows installer did so in order to make the installation go faster and smoothly due to the sevral reboots needed.
All you need to do now is to open the UEFI settings (what you 'wrongly' assume is BIOS) and change it back to Fedora (Grub). Then boot Fedora and run
sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
to update Grub and include the newly installed Windows.
