There are multiple ways to do this.
The other ASes could be sending a default route, or the router in AS100 could just have a default route configured.
The other ASes could just advertise their own routes through BGP.
The other ASes could advertise full BGP routes to AS100.
BGP has many factors which could play into the decision of which way to switch traffic destined to another AS. This is the subject of entire books, and it is beyond the scope of this site. It is far more complicated than IGP routing protocols, and it may involve many steps to determine the best path. Often is just boils down to how many AS hops away it is to get to the other AS.
You may be confused about the role of iBGP. The distinction of iBGP and eBGP is whether or not the neighbor is in the same AS. An AS will almost always have more routers than just the routers connecting to other ASes. The routers internal to the AS would have the same AS number as their neighbors, so they would use iBGP.
It would also be a huge discussion about how design within an AS. Again, you could have default routes, full or partial routing tables, a combination, etc., or a mix of IGP and iBGP (which is involved because you could use a full-mesh, route reflectors, confederations, etc.).
What BGP neighbors send each other can be controlled. It could be full routing tables, or it could be whatever the AS owner decides is appropriate. there is no one answer to this question.
Best Answer
When in doubt, you should check the RFC (RFC 2453, RIP Version 2, Section 3.10.1, Triggered Updates):