Can someone explain me how memory is allocated when a database is started? Explain me the memory splitting for each part like buffers,broker,server and for user ? Does the default memory allocation is same regrdless of various hardware?
Have you done any reading of the manuals and tried to find the answers to these questions yourself. The questions seem very broad. People are much happier answering a question like "I read about X but don't quite understand Y" than "Explain everything about Z".
The database inventories all of the configuration parameters that were specified and the default values for those that were not, and calculates how much memory will be needed based on the size and number of all the internal memory-resident data structures. Then it creates shared memory segments that have at least that much space plus a bit extra for various things that cnnot be calculated precisely. Once all the shared memory segments have been created, all the data strucutres are allocated within those.
I'm not going to explain the details of this because it changes from release to release and I am too lazy to do it.