What are the things that I have to know in mit for back end development of the app?
For example, If I am building an app that demands users to enter (and interact each other) like this mit ai2 community forum, then what are the things I have to know (in mit ai2 app development)?
You wouldn't use App Inventor to do the back end development. You would need another framework for that. The important thing would be to define an API between your back end and your mobile app built with MIT App Inventor that you would then leverage via a component like the Web component to communicate between the two platforms.
How can we make use of tinydb in this? I heard that whatsapp or telegram never stores user data with them. Can we create an app like that using mit ai2?
Okay. Il look into it. But, instead of cloud db, how can we make use of tiny DB? How whatsapp or similar apps are maintaining user data without storing them in their end?
To be able to interact a real time database is needed so the users can INTERACT.
Perhaps a TinyDB can be used for users to save part of the interaction but the app itself has to use a real time database all users can use in common. 
Can anyone explain how the apps like telegram or whatsapp is functioning without storing user data with them?
who is them?
And what kind of user data you are refering?
User's data means chats, media files in chat etc. I guessed that they are making use of user's device itself to store user data in some way.
You need some file server, and database for the chat message
I shall read about how their apps work in google and come back.
I saw these paragraphs from the website url given below.
Daily optimization to WhatsApp provides updates to general maintenance, deleting disappearing messages, chats, and information for Google Drive backup. We do this by automatically backing up your messages daily to your phone’s WhatsApp database. Your phone will store your chats to your phone’s WhatsApp database or SD card depending on your device settings.
Your phone’s WhatsApp database stores your complete chat history in order to recover them in the event of corruption. Daily optimizations to your phone’s WhatsApp database are automatic and can take as little as ten seconds, so you don’t have to worry about downloading updates manually. You also don’t need to be online during optimizations, and you don’t need to activate specific settings to enable it.
We recommend using Google Drive to backup a full history of your chats and media files.
https://faq.whatsapp.com/947033946530087/?cms_platform=android
So what is your concerns?
If you want to check how Whatsap store their data, you need to ask in their forum, not here.
If you want to make an app like Whatsapp, please start with an easy one, like chat app first.
So, If whatsapp preserves user data on user device itself, then is it tinyDB? Or, Is there a way to make a chat app in mit ai2 to store user data in tinyDB but works like whatsapp? You can reply as how you reply to an advanced developer eventhough Im a newbie. From your reply, Il collect clues to know more.
yes, but better with SQLite.
Can you please explain the steps? I shall write few steps below,
Step 1: Creating front end design. --- I can do this uing layout, buttons etc.
Step 2: Doing something to send and receive message. --- I can do this if i watch few youtube videos.
Step 3: Adding Tiny DB as database. --- I can do this as well with the help of youtube videos.
Step 4: Is there a way to encrypt and decrypt messages using mit ai2 extensions? Please explain this.
Step 5: Explain other missings steps as well.
Search the community, you will get most of your answer:
Here is a Firebase Firestore Guide that seems to do everything you want to do...