When you publish content in WordPress you are usually working in a Page or a Post.
When you create a post in WordPress, it is best to assign a category to it. Create a new category item as appropriate. In your custom menu you can then add a link to a category. Normally you would add static pages to the main menu.
By default, upon installation, WordPress’ home page is the blog. For this website, I have changed the settings for that to create a “brochure” site that still retains a blog but puts that blog as a menu item in the main menu at the top of the website.