If you want to relate all items that have a given tag (or more than one tag), you can use the Zutilo plugin: select the tag(s) you want in the Tag Selector, then select all the resulting items in the main pane, and finally right-click to Zutilo: Relate Selected Items. Each selected item will be related to every other selected item.
https://github.com/wshanks/Zutilo

A Saved Search for the same tag(s) would show all those items, as well as being updated every time you select it.