As far as i understand what you are asking is if BGP works with different AS, the answer is YES BGP is meant to provide communication between different AS. The internet nowadays is based on BGP. the current version is 4. It aslo doesnt matter that what IGP is the AS running. BGP performs interdomain routing in Transmission-Control Protocol/Internet Protocol (TCP/IP) networks. BGP is an exterior gateway protocol (EGP), which means that it performs routing between multiple autonomous systems or domains and exchanges routing and reachability information with other BGP systems.
Yes you can peer with a lot of BGP neighbors via one interface as soon as there is IP reachability between each of them, you can runn EBGP or IBGP with a lot of peers regarless the peers address the only condition you must take care about is there is IP reachability between peers because BGP establish TCP session with each peer (IBGP-EBGP) and based on networking basics the TCP flow (Source & destination) must be reachable, the below URL shown good case study that can help you to under stand a lot of BGP commands.
The above example shown that there is one IBGP session and 3 EBGP sessions, the IBGP session using the loopback interface as source when establishing the BGP session (Source of the TCP session), IBGP peers not need to be directly connected as you may run partial mesh IBGP session between edge routers which is not directly connected or between edge routers and Route-Reflector, but with EBGP session there is a rule say that both EBGP peers must be directly connected because its External BGP so sure you will not peering with one provider through another provider the logic is that you peering with EBGP neighbor via one interface or multi interfaces but MUST be direclt connected, BTW the above EBGP configurations you using one ip of /25 in the same interface that EBGP session running and all neighbors is directly connected so you doesn't using update-source loopback # but in the same time you still can using loopbacks in peering with each other, generally loopback interface is useful in many other scenarios such as load-balance, load-sharing, RID etc., finally EBGP peers ip address must be directly connected and if you do not need to use the interface address you can use Loopback it?s optional but with one condition that is you using Loopback in EBGP scenario you MUST use ebgp-multihop command.
For example i will use the below configurations with this senario:
RouterA (RR-A)have 2 interfaces F0/0 S0/0 and both interfaces connected to my network to runn IBGP with my edge routers so i will use the loopback interface of RR-1 because i need to peer with all edge routers and if i'm using one interface IP as my BGP router id once the interface went down the bgp session will went down which is not perfect because i'm still have another interface F0/0 is up so using the loopback interface as BGP session ID in this case is perfect.
neighbor 192.168.99.1 remote-as 100
neighbor 192.168.99.1 update-source loopback 1
NOTE: the loopback ip must be reachable from other peers side
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