Right now it’s a manual process to build the tarball. We basically check out each repo, run whatever build command is specified in the README, and then copy the artifacts into a temporary location.
To build the tarball, we unpack the OpenSearch tarball and run opensearch-plugin install file://path/to/zip for each plugin, then create a new tarball that rolls up the OpenSearch + installed plugins filesystem.
We’re working on automating this stuff, so you should see scripts start to appear in the opensearch-build repo over the next few weeks.