This app was mentioned in 9 comments, with an average of 1.11 upvotes
Inability to register is an issue with Firechat. Try a different app like Zapya which doesn't require registration and works without internet connection via WIFI direct and bluetooth for joining.
https://play.google.com/store/apps/details?id=com.dewmobile.kuaiya.play
It does both. Personally tested on two different devices with WIFI and bluetooth enabled but without internet connection. Created a group on one device, scanned QR code on the second device to join group, messaged back and forth and shared pictures.
https://play.google.com/store/apps/details?id=com.dewmobile.kuaiya.play ⚡Offline Chat Chat and share media with friends nearby without having to use Internet or data.
Use Zapya. When both phones are connected with each other, you can see what new version apps are present in the other phones and on granting permission you can get them all.
I think Shareit has it too but I am not that sure.
Try Zapya, available both on W10/W10M and Android, uses Wi-Fi Direct (so no server in between).
https://www.microsoft.com/en-us/store/apps/zapya/9nblggh3v9m6
https://play.google.com/store/apps/details?id=com.dewmobile.kuaiya.play&hl=en
Where in China do you live? A city? Not everyone China has limited internet connection that's just not true. I am sure you know that there is a huge disparity between different regions. Some places in China barely have any sort of technology at all. Contrast that with the major cities. I am not talking about censorship. I mean like actual poor connectivity to the point you cannot transfer files or download apps in any reasonable time. Also Apparently in China it's called Kuaiya.
https://www.techinasia.com/china-startup-file-transfer-app-zapya-gets-series-b-funding
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Like I said it is used mostly in poor developing countries who have very poor ISP infrastructure. Unless you live in countryside China, or cannot afford unlimited data, there is not much reason to use. In places where some people can afford to pay for data, while some can only use it sparingly it is very attractive as you can transfer files and apps without using up your data.
" While the app was initially conceived for the Chinese market, it has spread to neighboring countries such as Myanmar, Pakistan, and India and is available across multiple operating systems. "
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https://www.researchsnipers.com/top-10-google-play-apps-in-pakistan/
https://www.mmtimes.com/news/downloading-popular-smartphone-apps.html
http://marimundo.com/apps-for-cuba/
https://play.google.com/store/apps/details?id=com.dewmobile.kuaiya.play&hl=en_US
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Seems to check out. Also Gandhi visited their offices and they have a very large team so they must be doing pretty well.