The official description of $GITHUB_STEP_SUMMARY reveals a bit of the implementation details:
> The path on the runner to the file that contains job summaries from workflow commands. This file is unique to the current step and changes for each step in a job. For example, /home/rob/runner/_layout/_work/_temp/_runner_file_commands/step_summary_1cb22d7f-5663-41a8-9ffc-13472605c76c.