Well, I came across another laptop that hadn't/wouldn't upgrade to 21H1. I didn't want to take the hard way again.
If a PC automatically offers 21H1, the installation is QUICK. So I set out to find how to force that laptop to upgrade in the same manner as if it were automatically offered.
And I found it.
It seems that 21H1's functions were actually delivered in previous months cumulative updates. The "Feature Upgrade" that is automatically offered just flips some registry switches and reboots.
That "Feature Upgrade" is simply KB5000736, an enablement package. This enablement package is only available for devices running versions 20H2 and 2004.
If you have a device that isn't offering 21H1, just download KB5000736 (64-bit) and run it. Incidentally, when I clicked on that link, nothing happened. I had to right click and choose "Copy link address" and then paste that link into another browser tab.