When pages have “parent” pages or belong to a certain category explicit backlinks are added manually. Add the appropriate parent or category tag by referring to it in the page or place it at the end of the page. The backlinks can be searched for, automatically. Just click on the header to list all other pages linking to your current page. This is just a plain ol’ regular search.
This usually looks like the end of a page:
The reason this is not all automated is because a) it would require the maintenance of a separate link database, and b) in a double linked graph there may be potentially a lot of irrelevant pages pointing to the current page, so you do not always want to list them. Thus, the manual strategy of adding category tags or parent pages at the end.