What network information is, without additional configuration, shared between two iBGP neighbors by default?

iBGP works by exchanging routing information between two or more routers within an AS. Each router sends its own routing table to its neighbors, which contains information about the networks it knows and how they can be reached from that router. By default, iBGP neighbors only share BGP routes learned from eBGP neighbors, which are routers in different ASes. This is because iBGP assumes that all routers within an AS have consistent internal routing information provided by an IGP, such as OSPF or IS-IS. Therefore, iBGP neighbors do not need to share IP address information of loopbackinterfaces or directly connected interfaces, unless explicitly configured to do so by using commands such as neighbor update-source or network.

References: iBGP Ultimate Guide | How iBGP Is Different From eBGP, Ericsson IP Networking - Routing Protocols