Most of us have the Git information already entered. What most people are likely missing is the fact that a comment must be entered in the textfield at the top of the screen. So, after selecting Commit... from the Integrate menu, even if you Stage the changes, you need to enter something in the comment field. The fact that it's required appears inside the field... but it's nearly impossible to see if you're in Dark Mode: Thanks to this post for revealing the answer: https://developer.apple.com/forums/thread/757483?login=true

I'm not sure what ended up working, maybe force restarting the iPhone (just shutting it off and then restarting it didn't work) is what ultimately did it. Along the way, I used the Devices and Simulators window to remove all the installed apps, unpair iPhone, re-pair iPhone, and uncheck Connect via Network, while also rebooting both Mac and iPhone. When I originally checked the DandS window, the iPhone had a yellow warning, which I wish I'd copied down before restarting everything--something to do with icons, I think. My iPad had a red warning saying that it was locked and to reconnect it... even though it was connected. Did the same unpair/re-pair/etc. things with that as with the iPhone and eventually it worked as well.

Looks like I had an old version (2.1) of SF Symbols installed, not the newest one (3.2). The new installer installs the correct version of the SF fonts on its own.