Now I'm baffled but happy. As an experiment, I decided to re-do the install with the drive installed internally.  When I booted up my install media (thumb drive with Ventoy booting a Windows ISO file), I tried Windows' repair option to fix problems in booting.  I had tried it before, but it's quick so I tried it again.  Again, it reported that it was unable to repair my install.  The only option was to restart.  So I let it restart without pressing F2 or F12, expecting to get back to my install media.  Instead, I got the Windows I had installed last night. Wow. So I removed my media, and rebooted.  Wow! I have no idea what I fixed by doing this, but Windows now boots just fine.  I'm going to call this an "answer" although it raises as many questions for me as it provides answers.  But I can go on about my work with those questions still open.
I've had Windows on this Optiplex 990 for a long time, but it was installed MBR-formatted.  Not a problem until I wanted to use bigger disks.  I was able to change to UEFI, boot a windows installer, and get Windows running on a GPT-formatted disk. Until I rebooted. Now the BIOS cannot see a bootable disk (other than the thumb drive I used to install it).  I do not know what to put in the Boot Options in the BIOS. How do I get this thing going again?  I'm also going to want to install Xubuntu on a second drive, and may need instructions for that too.
Setting (BIOS F2) Optiplex 990 UEFI Boot Options.