If you want certain scripts, styles, html to be added to the entire site, please use 'AdvancedSettings' sheet. Don't follow this page for applying Global settings. These instructions are only to apply scripts, styles, HTMLs to the <head> tag in specific detailed pages.
For scenarios where you need to embed specific html, css or script inside head tag of a specific detailed page, follow below steps.
Running a specific script on a few pages
Applying specific styles for a few pages
Adding required SEO tags to specific pages
Note that Templates are supported for adding the required content in Detailed Pages head Tag. See here for more info. This will make it much easier to add content to <head> tag in thousands of detailed pages by just creating a single template and using it on each page by just adding a single line template code.
Steps to follow:
Go to 'DetailedPages' sheet in the Google sheet connected to your project.
Go to the specific row (row corresponding to the page you want to update)