For JNI please refer the documentation on Oracle's website.
For everything below that, the best approach is to just dive into the AOSP code(https://source.android.com). There is lots of documentation there as well. But the source code still gives you a lot better insight into the workings of the system and what the top layer expects from the bottom layer. As they say, "Read the source Luke!" :)
200,000 users for group chat. Unlimited users for voice chat (like clubhouse). 30 users for group video calling. That limit will be increased later. Flawless sync across platforms. Secret chats - the e2ee chats self-destruct between users and stay on the device where it has been initiated. Robust third party clients-that add more functionality to the official app. (I use Utyagram; Plus is another popular mod, but is closed source). Unlimited users for channels - that work as broadcast lists. The lists goes on and on.
1. Due to communist style of functioning, setting up a business is near impossibility. I know friends who moved their business out of state since its marred with strikes.
https://www.hindustantimes.com/india-news/18-days-after-nri-...
2. It is fast becoming Islamic state of India.
https://www.theguardian.com/world/2016/nov/29/isis-recruiter...
https://m.economictimes.com/news/defence/kerala-coast-on-ale...
https://www.indiatoday.in/india/story/nia-conducts-raid-in-t...
3. Infrastructure seems to be dated (single lane road for most part of state) and floods wreak havoc every year.
Tagged as "God's own country" I expect much better from Kerala. However, I always find cliched "most educated" state remarks and self-pat on back without perusing its problems and what is happening to state.
When you say features, which ones do you mean? Do you mean the user interface?
Also, I advertise that site any chance I get, and judging by your reaction I may have overstepped, so I apologize.
Hopefully someone else seeing it will gain something useful from it, still.