You generally open an .idx file directly. You let the parent application use it.
In this post, we’ll break down exactly what an .idx file is, the different ways it’s used, and how to open (or rebuild) one. what is .idx file
Because .idx files are "index" files, they are usually meant to be read by a specific program rather than opened directly by a user. What are .idx files? - Microsoft Q&A You generally open an
If you are curious about the contents, you can try opening the file with a basic text editor like (Windows) or TextEdit (Mac). the different ways it’s used