The end result of this post is that what I’m experiencing is indeed part user error, part unexpected behavior in pavucontrol in XFCE, and part bug for the Telehealth software provider.
The main problem is the choice of words used in pavucontrol. The check mark setting titled “Set as fallback”, really means “Set as default”. And now I’m dimly remembering having this same issue many years ago when trying XFCE on a different Linux distribution. 
See the following link:
https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/DefaultDevice/
I don’t know how they can call fallback and default synonyms.
Unless I’m misunderstanding how this word is used in this program, dictionary references don’t match up with that wording.
The fix for me was after plugging in my webcam, to open pavucontrol and set the webcam microphone as Fallback. Then when I logged into my telehealth session through firefox, it saw that microphone as the first choice and all worked well. Once this was done a few times, pavucontrol remembers this setting for Firefox as I understand it, and I don’t have to check that setting anymore.
The unexpected behavior is that in my prior Linux distribution of Linuxmint Cinnamon that I used for many years, I never encountered this issue. And as mentioned in my prior comment, Manjaro KDE on my other computer does not either. When there is no audio inputs connected and you plug one in, it automatically sets it to default.
The bug with the telehealth software was reported to my healthcare provider who said they passed it along to them.