Changes made when customizing theme do not show up in HTML in pages section

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I have been using customize on my theme to add and edit sections on my site pages. However, I have noticed that when I go to pages instead of going through the theme that none of what I have added is showing up. I wanted to use the HTML editing feature to add anchors to sections of my about page and then link those anchors to a submenu under about in my main menu, but when I go to the HTML on my about page under pages none of what was added to that page through the theme customization is showing. Am I missing something? Can I only use one or the other?