To set a custom environment variable for a single workflow, you can define it using the env key in the workflow file.
Example:
env:
DAY_OF_WEEK: Monday
Note: The scope of a custom variable set by this method is limited to the element in which it is defined. You can define variables that are scoped for:
The entire workflow, by using env at the top level of the workflow file.
The contents of a job within a workflow, by using jobs.<job_id>.env.
A specific step within a job, by using jobs.<job_id>.steps[*].env.
Reference:
https://docs.github.com/en/actions/how-tos/write-workflows/choose-what-workflows-do/use- variables

You must store workflow files in the .github/workflows directory of your repository.
Reference:
https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax

The use of self-hosted runners is required when your workflows depend on specialized hardware configurations that are not available through standard GitHub-hosted runners. While GitHub offers "larger runners" with GPU support for certain paid plans, self-hosted runners provide the most flexibility for highly specific or proprietary hardware needs.
Scenarios Requiring Specialized Hardware
Specific GPU Models: Workflows for AI/ML training or intensive graphics rendering that require specific NVIDIA or other specialized GPU architectures.
Alternative CPU Architectures: When you must build or test on specific ARM processors, legacy x86 32-bit systems, or other non-standard architectures not supported by GitHub's managed pool.
High-Resource Requirements: Tasks needing massive amounts of RAM (beyond 256 GB) or high-core counts for extreme parallel processing.
Custom Peripherals: Workflows that need physical access to hardware connected via USB, PCIe, or other local interfaces (e.g., embedded systems testing or hardware-in-the-loop).
Reference:
https://docs.github.com/en/actions/how-tos/manage-runners/self-hosted-runners/use-in-a- workflow

Variables reference
Find information for supported variables, naming conventions, limits, and contexts in GitHub Actions workflows.
Variables include:
* GITHUB_REPOSITORY
The owner and repository name. For example, octocat/Hello-World
* Etc.
Reference:
https://docs.github.com/en/actions/reference/workflows-and-actions/variables