On macOS, 'sandboxed' apps (which conform to Apple's stricter security protocols, similar to those on iOS) will indeed put all of their files inside ~/Library/Containers (or ~/Library/Group Containers for data that they might share with other apps).
The subfolder for the app is usually given the app's name, or bundle ID (e.g. com.company.appname.)
So you shouldn't need to use lsof, just browse the Containers folder for the app's named folder.
You'll see that the contents are a mix of "real" folders, and aliases to allowed locations, like ~/Library/Fonts/ or the Desktop folder.
However, non-sandboxed apps may store their files in any sub-folder of the user library directly, most frequently:
Preferences
Application Support
An app's files and folders might use the app's name, or the bundle ID in the filename. Equally, it might not have any recognizable reference to the app in its name.
I would also be wary of 'app cleaner' apps, as they can miss files -- or worse, delete the wrong ones -- just as easily as you can. Many have a bad reputation in terms of aggressive marketing and exaggerated claims, particularly encouraging the myth of the need for 'regular maintenance'.
For most apps, the total amount of user data is very small -- usually kilobytes. (Remember that on iOS, deleting 'app data' also includes any documents in the app's folder; which won't happen on macOS.)
If you delete the app alone, these files do nothing more than occupy a tiny bit of disk space. They don't 'slow your Mac down' or otherwise damage anything: they just sit there, un-read and unloved.
