I needed a reminder of where to look, and the first search result was in a reddit post on r/pop_os with a similar title. Since that sub is currently locked, I found my answer elsewhere. I decided it would be useful to post the info here too. I’ll write my answer below, and someone can write a better one if they have it.
(p.s. Thanks for keeping the reddit sub private; I think it’s the right decision since the major reasons for the blackout have not changed.)
Here’s what worked for me.
The most likely places for user-installed applications:
~/.local/share/applications~/.local/share/flatpak/exports/share/applications
The first of these is for .deb applications, and the second is for flatpak.
Other places:
This answer on ask ubuntu says the default search also includes
applicationsdirectories that exist under the directories included in the variable$XDG_DATA_DIRS.This flatpak github comment gives the directory above for flatpak, along with a lot of complicated explanation and other possible locations. If you need to know that, you should probably just read it there.