Hi Parth,
The Office Open XML markup references are documented in ISO 29500 in four parts. You can find the main reference for the markup language in part 1. Here's the link to download the documents:
https://www.iso.org/standard/71691.html
(click on the "download" hyperlink)
Along with that, you would also want to confer with the Open Specifications, specifically our standards based implementation notes and extensions documents here:
Word, Excel, and PowerPoint Standards Support
In this set of documents, you can pay close attention to MS-OI29500 and MS-DOCX, MS-XLSX, MS-PPTX and MS-ODRAWXML.
I hope this helps,
Best regards,
Tom Jebo
Microsoft Open Specifications Support