Later I noticed a taskbar icon showing that there were many windows spawned with the title "pick an app", as shown here:

Each one of these "pick an app" windows prompted to select an app to view a file with a certain extension, as follows:

Then for some reason the taskbar button and all of the "pick an app" windows simply disappeared. I can't think of anything I did to make that happen.
I'm guessing that these windows were spawned because of the files with different extensions listed in the Everything results list. It seems that there was an attempt to open at least some of the listed files, and Windows needed to know which app to use to open each file. I don't remember double-clicking or doing something to initiate a File Open operation. I tried highlighting multiple files in the results list and then manually initiating a File Open, but even in this case, only a single "Pick an App" window was spawned. So maybe the spawning of multiple "pick an app" windows was something not done by me, but somehow generated by Everything? Perhaps it's not a big problem since the taskbar button and spawned windows went away, but I thought you might want to know.