I am an experienced C/C++ programmer and have worked with assembly and many other programing language and I want to start a project as a learning process. I want to try and run a simple custom os on the iphone or ipad. What knowledge would I need to do this, and how does the iphone or ipad bootloader load the os and how could I modify it to load a custom os?

Im not really sure what to ask here so I really just need to get as much information as possible so I could ask some more informed questions to get my project started

Thanks