When we create a report it is generated in XML raw data format.
When creating the HTML report we apply a transform using an XSLT.
For example for the requirement report the XSLT is for example :
If you need to modify it you just need to copy requirementHTML_Complete_with_stats.xslt file on the server (in the export/xsl/folder) and adapt it to your needs.
You'll need some XSLT transform knowledge to do that but it's not that complicated.
If you need to see how the XML raw data looks like, you can simply generate a report and select "(XML) Raw data".