Hello Nihar
By definition, iBGP advertises prefixes between BGP routers within an AS. Similarly, by definition, eBGP advertises prefixes between BGP routers on different AS’es.
So iBGP is responsible for learning the routes within the local AS and delivering them to iBGP routers that are on the edge of the AS. Those edge routers, also have peerings with BGP routers in other AS’es using eBGP. They learn all of the prefixes inside the AS, and will advertise them using eBGP to other AS’es.
So why is eBGP used only for communication between AS’es? Well, it’s simply a matter of design. This is done by definition.
I hope this has been helpful!
Laz