What kind of AI does Minecraft mob use?
Behaviours are put into reusable modules. Each mob gets assigned a prioritized list of modules. For example, if a mob is attacked it's more important to fight back than randomly strolling around, therefore the attack behavior kicks in.

Is the algorythm conplicated?
No, the trick is to keep things simple and in modules and avoiding cross behavior dependencies.

Has it many lign of code?
Not sure.

Can you explain it in the simplest?
See the first question.

What do you think abouts AI in general (outside of video game)?
AI is really cool, in a way it's the study of ourselves and something greater. I think we are very lucky to be living in this time since we are going to see a lot cool stuff happening the coming decades, everything from self driving cars to brain simulations. At one point video games will get this kind of AI as well. If you want to have a cool future job your should definately study AI, you will be needed (unless the ai is doing all the work already).

Good luck with the assignment.