How do I copy the HTML for the header and footer? I can not identify specific HTML in the source code. I am using the header and footer to wrap IDX pages with my branding.

The HTML for Wix site pages is not available, the HTML cannot be modified, and it is not possible to embed HTML into a page.

An HtmlComponent can be used on the page (depending on the embedded code). See the article Working with the HTML Element in Wix Code for details.