How to find ipsw file on macOS Catalina

It sounds like you are trying to retrieve the ipsw file directly from the ios device because the device has already downloaded the file internally. There is no way to retrieve the ipsw file version from the iPhone itself - if you even could - it may not be compatible to work through the Mac. Instead you will need to redownload the file on the Mac instead

While downloading the file through Finder, Open terminal:

%lsof | grep ipsw

This command will show you the open ipsw file (which is being downloaded).

The path to find the downloaded ipsw file is:

/Users/Username/Library/iTunes/iPhone Software Updates/