Also,
That quote suggests that writing operating systems for mobile phones it not what you do 8 hours a day, 5 days a week. You have been given answers by a few members with a lot of experience in that area. Those answers range from "impossible" through "incredibly difficult" to "impractical". Either way, it is not a task one person can take on, "as a way of learning things".

Where do you start? Spend a few years studying computer design, a few more years writing software, enhance that with some system programming and then you will have a rough idea.