Start with Policy To Policy. That graph answers the business question: "Which Apigee policy concept becomes which Flex Gateway policy?" For example, Apigee IP Allowlist maps to Flex ip-allowlist.
Then read Source Files To Policy. That graph answers the implementation question: "Which files make up the Apigee side of that policy?" For IP allowlist, the Apigee side has JavaScript logic, a JavaScript policy wrapper, and a forbidden-response policy. Those three source files feed one Flex YAML target.
The arrows are expressed with Vyasa depends edges. A target node depends on source nodes. Read that as "this target was produced from these source inputs."