Is there any design document or data flow diagram available for backtrader?
Can it be used(extended) for creating an live engine(trading for India marketplace)
You need to create your own Broker and data feeds. Some people have done it.
Also, is it possible to use it for trading strategies involving multiple assets
Yes. It's built-in.
That's up to you, not to backtrader. See https://github.com/Kismuz/btgym