Re: How to combine bitstreams (thrid party IP cores) to use it in main design?
It depends what the IP is.
If it is a complete design, then yes, it will be a bitstream. This cannot be combined with anything else as it would program the entire chip.
But you can get IP in the form of source code, encrypted source code, netlist (synthesis or post fit) which can all be compiled into a design. Here, you dont get provided with a bit stream, you compile the provided source.