Pages in this site link to each other by name. In the editor, type [[ and choose a page, or write the name yourself:
[[Buttons]]becomes a link to Buttons.[[Buttons|the button page]]changes the words: the button page.[[Buttons#Usage guidelines]]links to a heading on that page.
A name that matches no page is shown with a wavy underline, so it is easy to spot (and Content health lists it). Every page ends with a Linked from list of the pages that link to it, so nothing is left on its own. What changed lately is on the Recent changes page.