1. Get the GUID of the simulator that you were using

cd ~/Library/Developer/CoreSimulator/Devices  

This folder contain a file named device_set.plist.

• Open this file in a text editor
• Go to the section that lists all the simulators for the specific iOS version of interest e.g. com.apple.CoreSimulator.SimRuntime.iOS-14-5
• In the dictionary () scroll to the simulator you were using e.g. com.apple.CoreSimulator.SimDeviceType.iPhone-12-Pro-Max
• The entry below that key is to GUID of the simulator.

2. Find the application folder of the app

Go to the folder that contains all the application data folders from the iOS simulator of interest:

cd <Simulator GUID>/data/Containers/Data/Application/  

List all files sorted by date, most recent at the bottom

ls -ltr  

The last folder in that list most likely belongs to the app.
In there you will find e.g. the Documents and Library/Application Support folders used by your app.