In-chat apps

From the Delta Chat FAQ.

You can send apps to a chat - games, editors, polls and other tools. This makes Delta Chat a truly extensible messenger.

In a chat, using Attachment Button → Apps
You can also create your own app and attach it using Attachment Button → File

In-chat apps can not send data to the Internet, or download anything.
An in-chat app can only exchange data within a chat, with its copies on the devices of your chat partners. Other than that, it’s completely isolated from the Internet.
The privacy an in-chat app offers is the privacy of your chat - as long as you trust the people you chat with, you can trust the in-chat app as well.
This also means: Just like for web links, do not open apps from untrusted contacts.

In-chat apps are zip files with .xdc extension containing html, css, and javascript code.
You can extend the Hello World example app to get started.
All else you need to know is written in the Webxdc documentation.
If you have question, you can ask others with experience in the Delta Chat Forum.