Yup - you are missing a few things. Let me clear it up:
‘Open Distro for Elasticsearch’ is a distribution only. It takes Apache 2.0 licensed Elasticsearch & Kibana 7.10.2 and pairs it with Apache 2.0 plugins that round out the functionality giving you things like SQL, authentication, and alerting. In this case, the Open Distro team does develop the plugins, but uses the open source binaries of Elasticsearch and Kibana from Elastic.
There is an ongoing effort by this team to fork Elasticsearch and Kibana after the announcement that future versions would be licensed as non-open source software. This effort is still underway and hasn’t been released yet - see the ‘Stepping up: Elasticsearch & Kibana Fork’ category for updates.
Future versions of ‘Open Distro for Elasticsearch’ (under a new name) will use these forks, but not for a while. The initial releases of the forks won’t be production-grade software. In the interim, Open Distro will still use Elasticsearch & Kibana 7.10.2.
The build repo (attached to this forum category) is for building the artifacts for release. A lot of it is tied to the release process of Open Distro itself - you’re welcome to look around and contribute (it’s OSS!) - it’s just not a great starting point. Most folks use the binaries that are built by this repo (see the Downloads page).
Now looking at the context of your question (other OS’s / *nix / Illumos) there is another onion to peel
Today Open Distro uses pre-built binaries (linux, windows, macos) - will that work for your OS?
Once the forks are out (shortly!), you can build Elasticsearch and Kibana from source and might have better luck porting to a different OS. Either way, the build scripts for these forks will be in the repos for each individual project.
Hope it helps!