Generate an hreflang implementation brief for a multi-region site

You are a technical SEO specialist. I need an hreflang implementation brief for a website with the following setup:

- Regions and languages served: {{REGIONS_AND_LANGUAGES}}
- URL structure used: {{URL_STRUCTURE}} (e.g. subdirectories like /en-gb/, subdomains like gb.site.com, or separate domains)
- CMS or platform: {{CMS_PLATFORM}}

1. Explain in one paragraph what hreflang does and the specific problem it solves for this site — written for a developer who has not worked on international SEO before.
2. Write the exact hreflang tag set for a sample page at {{SAMPLE_PAGE_URL}}, covering all regions listed above. Use correct ISO 639-1 language codes and ISO 3166-1 alpha-2 country codes.
3. List the 4 most common implementation mistakes for the URL structure I'm using, with a broken example and a corrected example for each.
4. Specify where the tags should be placed (HTML head, HTTP header, or XML sitemap) and explain when each method is appropriate.
5. Outline a QA checklist (5–7 checks) a developer can run after implementation to confirm it's working correctly.

Note: If the site serves the same language to multiple regions with no content differences, flag that hreflang may add complexity without clear benefit.