I think you do not understand what a child theme even is. Given the thousands of articles about this online, I will let you google it.
Everyone has a different approach, but for me I always use a child theme. It is merely a way of enabling changes to the main theme which do not get ‘broken’ or overwritten when you update the main theme. You can add custom css style, and custom functions to a child theme.
Many plugins also like child themes… such as woocommerce, which expects you to put templates and other items in your theme folders - and so a child theme is best here.
Whether you use a child theme or not has nothing to do with headers, navigation or anything else you mention. Hence my suggestion that you google wordpress child themes. You could do worse than starting here: