Following up on @ronnqvist's advice - symbol graphs are output by the swift compiler for Swift, and the Swift DocC plugin does the pertinent orchestration for swift packages/libraries so that you don't have to fiddle with it when generating documentation. The underlying docc driver command takes one or more of them, along with the catalog location, when it's making the DocC archive - and I'd agree is the easiest place to start.
If you want to explore some existing symbolgraphs, you can dump them out from a swift project using the command: swift symbolgraph-extract, although there's also ways of getting the raw details straight from the C compiler. There was a forum thread about which to use a few months back.
The structure of a symbolgraph isn't strongly documented - is generally JSON, and the swift library that models it is GitHub - swiftlang/swift-docc-symbolkit: A Swift package for encoding and decoding Swift Symbol Graph files. - and there's documentation for it on GitHub pages from that repo : https://swiftlang.github.io/swift-docc-symbolkit/documentation/symbolkit/
Those docs, particularly Documentation and below, highlight the properties and hint as to how they're expected to interact.