What's a basic run down of how iBGP routing works?

I'm having a hard time finding details on internal BGP, mostly finding explanations for external BGP.

Say a user on ISP X is connecting to a server also on ISP X's network to download a file, what would the basic steps of this routing process be? I understand that in eBGP the autonomous systems will communicate the subnet reachability information with neighbouring autonomous systems and then send that information to the internal routers of the AS, but how does this work internally if both machines are using the same ISP in say the same region, as thats one AS (I think)?

I'll appreciate any help.