Sloth is a Mac application that displays a list of all open files and sockets in use by all running applications on your system. This makes it easy to discover which apps are using which files and sockets.
Sloth is essentially a friendly GUI built on top of the powerful lsof command line tool, whose output is parsed and shown in a searchable, filterable table view with some convenient user-friendly functions such as revealing files in the Finder, killing a file's owning process, displaying file info, filtering by volumes, and so on. See screenshots below.
Excellent app! Reminds me a bit of fseventer that's been discontinued (or not updated for our more modern OSes). I didn't see a link in your original post. You can download it here: https://sveinbjorn.org/sloth
I will be using this to pull back yet another layer on the OS for my students as I train them to be Junior Mac Geniuses at my Help Desk.