How to add archived posts to pages/tabs?

I am confused as to how I can manage my pages so that certain pages display all posts that have a certain tag.

For example, if one of my tabs (pages) said “Countries” and then one of the pop-down tabs said “Bolivia”, how do I make every post I’ve tagged with Bolivia be archived there?

I’ve attached a link to a website that has this feature: http://www.bridgesandballoons.com/

Many thanks in advance.