Your “mini app” questions. Develop and show around the app you created.
You can send apps to a chat - games, editors, polls and other tools. This makes Delta Chat a truly extensible messenger. Just try it out now using the “Attach Menu”. Mini apps run in Delta Chat, Arcane Chat, XMPP Cheogram and Monocles and others.
But there is more: A mini app is just a zipped index.html plus some assets - really easy to create, esp. for casual devs. See the Webxdc Format for details and examples.