Is that very hard to process on today's computers?
For a programmer that has to ask the question - yes. For a programmer capable of writting efficient code and using a modern high end computer (that would right now have 120 physical threads) - not really. THe AI may be a problem, but then that can run on a cluster of machines.
Does every computer split up the job and do their own part?
Do you TRUST your players not to cheat? If you do not - can you answer that question yourself?
Generally you hire someone who has experience writing distributed systems. Generally this is way over the head of "a programmer asking question" and the level architects with real time experience come into the game. All thos bullets may sound heavy, but I proces a data stream here doing sometimes in excess of 100.000 updates per second, so - it is doable.