To get the example to work (i.e. to have one workflow wait for another to complete) you need two files. Both files live in the .github/workflows folder of a repository.

The first file would be set up as usual. This file will be triggered by whatever event(s) are set in the on section:

---
name: Preflight

on:
  - pull_request
  - push

jobs:
  preflight-job:
    name: Preflight Step
    runs-on: ubuntu-latest
    steps:
      - run: env

The second file states that it should only trigger on the workflow_run event for any workflows with the name Preflight and must be on your repository's default branch, usually main or master:

---
name: Test

on:
  workflow_run:
    workflows:
      - Preflight
    types:
      - completed

jobs:
  test-job:
    name: Test Step
    runs-on: ubuntu-latest
    steps:
      - run: env

This more-or-less the same as the example from the GitHub Actions manual.

As you can see on the actions page of my example repo, the Preflight workflow will run first. After it has completed, the Test workflow will be triggered:

When you try this out, you will find that the Test workflow always runs the code and workflow configuration on your default branch, usually main or master.

This is because, (quoting from the manual):

This event will only trigger a workflow run if the workflow file is on the default branch.

This means you'll need to develop workflows triggered by workflow_run on your default branch, which is a little unfortunate.

This also means that if you need the "Test" workflow to run against the code of your branch or pull request, then you'll need to do more work as described below:

Every actions is run with a set of contexts. The github context holds information about the event that triggered the workflow. This includes the branch that the event was originally triggered from/for: github.event.workflow_run.head_branch.

This can be used to check out the origination branch in the action, using the actions/checkout action provided by GitHub.

To do this, the Yaml would be:

---
name: Test

on:
  workflow_run:
    workflows:
      - Preflight
    types:
      - completed

jobs:
  test-job:
    name: Test Step
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with:
          repository: '${{ github.event.workflow_run.head_repository.full_name }}'
          ref: ${{ github.event.workflow_run.head_branch }}
      - run: git branch
      - run: env

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run

The workflow_run event only works on the default branch.

instead of using workflow_run, look at using reusable workflows. Then in your code quality workflow, if everything works there, you would call a reusable workflow to do the testing.

Use the /repos/{owner}/{repo}/actions/workflows/{workflow_id}/runs endpoint to list the runs of a given workflow ID in a given repository.

For example, to get all of the runs of the CI job for the rust-lang/rust repository, use https://api.github.com/repos/rust-lang/rust/actions/workflows/817415/runs.

This is now available in organization settings > Runners > GitHub-hosted runners. URL looks like https://github.com/organizations//settings/actions/hosted-runners. Screenshot: