Is there a difference between launching an app from /Applications vs /System/Volumes/Data/Applications?
Treat them as identical. There is no functional difference in finder or another program opening either path to the same Application bundle. You have only one Spotify app despite it showing up at two distinct POSIX paths.

It is not a sym link but a hybrid combination of two filesystem links that is presented as the path /Applications on Catalina. 

I would say the closest analog would be a hard link on OS prior to Catalina, but firm links are new to Catalina and are not the same as symbolic or hard links. 

You’ve chosen the strangest folder in existence on Catalina since it’s a hybrid / illusion as named in the most superb technical discussion on this idea I’ve seen:


  The old folder containing main apps, which used to be /Applications, now contains a mixture of apps from two different folders, /System/Applications, which is on the System volume and contains Apple’s bundled apps, and the Applications folder on the Data volume, which would otherwise be found at /System/Volumes/Data/Applications. When you install your own apps, they automatically get placed in the latter folder, as it’s the one which you can write to. When you view the /Applications folder in the Finder, though, you are actually seeing the combined contents of both folders, an illusion which is created by the firmlink at /Applications and the Finder.



https://eclecticlight.co/2019/10/11/macos-catalina-boot-volume-layout-revised-for-10-15-release/


Apple technical documentation is also plentiful, but not as detailed or readable as the Eclectic Light blog post.


What&#39;s /System/Volumes/Data?