The correct statements are Default environment variables use uppercase names only, Default environment variables cannot be read through the env context, and Default environment variables are available to every step in the workflow.

GitHub Actions sets many built in environment variables such as GITHUB_REPOSITORY, GITHUB_REF, and GITHUB_SHA. These are provided in all caps by the runner, which is why they are consistently uppercase.

The env context exposes variables that you explicitly define in the workflow, job, or step, or that you write to the environment file during a run. The built in defaults are not part of that context, so you do not read them through env. You access them directly in the shell for the runner, for example with $GITHUB_REF in bash or $env:GITHUB_REF in PowerShell, or you use the github context in expressions when you need similar values.

The runner injects these defaults for each job, and every step in that job inherits them. This makes them available throughout the workflow steps, although some values can vary by step or job when they are intended to reflect step specific details.

Default environment variables must be referenced using the $ expression syntax is incorrect because they are ordinary environment variables in the runner process rather than values in a variables context. In expressions you typically use other contexts such as github for repository and ref information, or you expose your own values through env if needed.

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