In Briar 1.5.15 on Android when you go to Settings there is an option to “Share this app offline” (next to “Share download link”).
It opens “Share this app offline” screen with the text "Share this app with someone nearby without an internet connection by using your phone’s Wi-Fi.
Your phone will start a Wi-Fi hotspot. People nearby can connect to the hotspot and download the Briar app from your phone."
This starts a Wi-Fi Direct network and an HTTP server in this network from which the app can be downloaded.
The source code appears to be here:
It would be great if Delta Chat can implement the same feature to share its own .apk.
For a start, even the ability to save an .apk to Downloads folder would be great, then users can figure out how to share it when they are offline as long as they can copy the file to laptop and from laptop to the phones. Starting a Wi-Fi Direct hotspot likely requires additional permissions that Delta Chat does not require because it does not use bluetooth or Wi-Fi Direct normally, so this is an overkill, and user can start a hotspot manually. Sharing the .apk via HTTP may be useful, but not really necessary for a start.