In all of these apps, every change you make updates the URL, instantly saving your progress in an easily sharable link. You don’t have to create an account or log in. In fact, you couldn’t if you wanted to because there’s no database! These apps are completely static.
What a fun idea. I've obviously seen (and have used) URLs contain state information in the past, such as user preferences (light/dark mode or animation), but building an entire game using nothing but URL parameters is pretty slick. The tip of encoding into base64 is great, too, as that really maximises what you can do with the space you have, though it does hurt extensibility (and readability) for your users.