Look into the StyleRef Field. That lets you have a single header that has different text on every page depending on the content of the page.
You can have a bottom border for the paragraph. This would best be done as a modification of the Header style.
By the way, when you type the thee hyphens --- and press Enter, what you get is a bottom paragraph border created by AutoFormat As You Type.

See also Word Doesn't Know What a Page Is by Daiya Mitchell, MVP( https://wordmvp.com/Mac/PagesInWord.html).