How to remove metal ring on flex conduit?
It's threaded into the flex conduit. it has ridges on the side of the fitting that match the pitch of the spiral of the flex.

In reality they are not really using that correctly, they are rigging it up using parts that are intended for a different use. Here is the actual assembly that it is part of (the photo is of a stainless steel version, but it's the same concept):



The part you are looking at on yours is only the chase sleeve, shown in this photo to the left of the blue ring. It is INTENDED to go into a fitting that is sealing against the seal-tite flex. They are using only that sleeve and using it kind of backward.