iBGP running only between BGP border routers?

        I was studying about the BGP protocol and I had this question:<br />

Let’s say we have an AS having both BGP border routers and internal routers . I know that the border routers should form a full mesh to have iBGP sessions and exchange external routes (learned by eBGP).
I was wondering how the internal routers are going to learn about these external routes.
Do the internal routers also establish iBGP sessions?; do the border routers redistribute the external routes through IGP protocol?; do the internal routes have static (or not static) default routes to border routers (but how to choose which one)?
Does anyone have an idea how the current network operators deal with this issue?