Building a fully functional multiple user chat app is extremely complicated. You should search this forum for links to the simple chat app example created by the thunkable staff to get started.
To build a multi user chat app with notifications, you will first need to study and understand the following Thunkable topics:
- Cloud variables
- JSON objects, properties, and arrays (lists)
- Cloning and create blocks
- Firebase authentication (sign in)
- Firebase realtime db listeners
- Web API calls
- One signal push notifications
- Asynchronous block management
The forum has a lot of good examples for all of these features, as well as the thunkable documentation.
After you are comfortable with all of these features, you can start putting them together for your app.
As you progress, try to ask very specific questions in the forum. The more specific your question, the better answers you will get.