I downloaded the Xcode 13 beta and waited through its lengthy installation process…
…and when I launched it once it had been installed, something was wrong. The SwiftUI Canvas preview screen didn’t work, and there were no Simulators available.
And then I remembered that this happened to me with the beta edition of the previous Xcode version. The Canvas makes use of the Simulators, and both work properly when you reboot the Mac after installing Xcode, which appears to reload some necessary files.
Simply put, the way to fix it is to use the classic IT solution: Turn the computer off and on again.
I did just that, and Xcode 13 beta now has a properly-functioning Canvas and a full set of Simulators running iOS 15 beta.