How do I see the XML of my DOCX document?

It's a zipped file. Rename it ending in .zip to view it.

Working on macOS, and don't want to install any software to see the XML from your .docx documents? Just open up the terminal and:

cd path/to/your/file.docx
unzip file.docx -d file-content

As mentioned above, .docx files are "disguised" zip files, and unzip is installed by default on macOS. After using it, your file-content folder will contain the various .xml files composing the Word document.