I came across solutions on how to merge docx files using c#:
Append multiple DOCX files together
In this solution, he iterates through files and copies the body "outerxml" to a new document:
XElement tempBody = XElement.Parse(tempDocument.MainDocumentPart.Document.Body.OuterXml);
newBody.Add(tempBody);
This looks somethign specific to c# api. But I am using Ruby. So far I have been able to edit the docx file and make changes to it by editing "word/document.xml". However, now I need to merge multiple docx files, and I would like to know if there is a specific xml file in openxml that encompasses an entire document, so that I can use that to copy into another document.