Ok so you can check:
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4.9. Adding Pages to a Course — Building and Running an Open edX Course - Cypress Release documentation for adding sections to a course.
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To include pages in the header you need to make your own fork and customize the open edx theme. Check this link 4.3.1.3. Creating a Theme — Installing, Configuring, and Running the Open edX Platform documentation .
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The registration page is most likely an MFE in your case and so check this link Overriding MFE styles