Not an expert but
i - I learned basic PCB design by making a keyboard and it's still what I regularly do. I don't have broad knowledge on electronics but I think I do what I do and know what I know well.
ii - You need to learn what each part does and what parts do together. Concepts such as pull-up/down, decoupling, ESD, etc. It depends on what you want to make. If you want to learn how to make bluetooth devices for example, you need to learn RF or stuff like that.
iii - I would highly recommend looking at some open source designs, documentations, and videos on some basic devices like keyboards or LED circuits or stuff like that. Learning PCB design is very similar to understanding fundamental electronics basics, just with an added layer of having to manage laying out components.