Visiting https://playground.wordpress.net/ opens a website that comes with a number of assumptions as explained in the docs:

Every time you visit the the official demo on playground.wordpress.net, you get a fresh WordPress site.
You can then create pages, upload plugins, themes, import your own site, and do most things you would do on a regular WordPress.
The entire site lives in your browser and is scraped when you close the tab. Want to start over? Just refresh the page!
WORDPRESS PLAYGROUND IS PRIVATE
Everything you build stays in your browser and is not sent anywhere. Once you're finished, you can export your site as a zip file. Or just refresh the page and start over!

It would be great to have an onboarding modal that explains that to new users in brief terms – perhaps a sentence or two and a few short bullets.

Also:

• It should be opt-out with a query parameter like ?onboarding=0
• Once closed, a cookie should ensure it won't be displayed again

The change involves React and belongs in the website project. PlaygroundViewport is a good starting point because it is the root component of the entire website.