To get the SQLite database for your app on all simulators on which it’s installed:
find ~/Library/Developer/CoreSimulator/Devices -name MyAppName.sqlite
(Xcode 10)
Open Finder and do Shift + Cmd + G to bring up the “Go to folder” dialog. Paste in the file path and you should see it. It will be garbled if you try to open it with a plain text editor. DB Browser for SQLite seems to work for me.