bgp is the protocol the whole internet uses to exchange routes. An autonomous system (AS) is a separate participant that announces the prefixes it can deliver. Neighbors (peers) choose the best path by policy.
This lesson is the bare minimum: two routers in different AS numbers,
a point-to-point /30 between them, an eBGP session. You configure it
through cmd-vtysh, read show ip bgp, and understand the session
state.
Each node already starts with the FRR daemons (zebra/bgpd/ospfd) up, through the init command in the YAML.