action.yml describes the action; which environment it runs in, what inputs it accepts, and what outputs it produces. action.js is where the actual program is written. It has two dependencies from GitHub Actions itself, which is why the node_modulesfolder is included in this repo.
The action is published to the GitHub Marketplace by creating a release. The release is tagged with a version number, which is used in a workflow file to specify which version of the action to use. GitHub automatically detects this as a publishable action, so it suggests to publish it to the Marketplace.
The metadata for this published action is described in marketplace.yml.
