How to Hard Code Custom menu items

Is there any way that i can hard code the custom menu items when first theme installed? I am creating a theme which will automatically make some common pages when installed. So I need to know if I can also add them to WordPress custom menu so client don’t need to add them manually?

Are custom inner theme folders in any way overridable by child themes?

Basically, I’m trying to override some files of the Uncode theme. But this is not a theme-specific question: it can be applied to any theme. The Uncode theme has a custom dir 'partials' (/uncode/partials/) that contains some layout elements. On my child theme I created a ‘partials’ folder as well (my-child-theme/partials), but the inner files seem not to be overridden. Is it normal? WP docs say that any file of the same name should be overridable, but is seems that’s not true. Am I doing something wrong?

How to show a single post on the front page but have normal paging?

I would like to have a single post on my front page (always the latest), but let normal paging work. So the front page has post 1, the next page should have post 2-11 (1-10 is fine too), then 12-21 or 11-20, and so on. I know I can change the number of posts depending on the context, but setting this to “1” on the homepage means the further pages also show only one post.