For being a project manager of any field, do you really need an experience as a person who already worked under manager?

To be successful Project manger you have to have an accumulated experience i.e.
you have to pass through a lot of problems , trouble ..etc.
learning from your mistakes , failed , ...to be successful person with excellent experience how you can handle & solve the problem

yes of course experience need to be a project manager if a person have already worked under the manager he has acheived experience

Project management and Product development are total different fields..
Its like people consider Sales & Marketing as one.
In Project management its a added advantage to have the Technical knowledge but nor necessary..

yes

yes

... and you will learn a lot from those managers that are the most despicable... those will be the ones that will allow you to grow as person and has a project manager

This is not necessarily. However, working under manager will be an advantage since it may enrich your managerial skills and experience.

yes