I'm researching various tunneling technologies and I have a fairly good understanding covering some of the basics. In a site-to-site VPN the edge routers could appear as only one hop away. I've also read that it's possible to make it appear as a flat network i.e. everything on the same LAN. This is the piece i am missing, can someone help me to understand how this is possible?

T.I.A. (Thanks in advance)