I tried unzipping the .xlsx file and could find some files in it. But I am not sure as to what these files contain and how they are related.
Where can I find the DOCS for creating spreadsheetML?
You can find the full specification of the Open XML file formats here. The specification contains a description of the files that are contained in an Open XML file package (such as an XLSX file), their contents and relationships.
There are also a lot of great articles and tutorials about Office Open XML formats at the OpenXML Developer Blog and Eric White's Blog.
For learning Office Open XML, I would recommend starting with this series of articles and this to learn how SpreadsheetML (the XLSX file format) works.