If you have set a static front page, and defined a ‘posts page’, WordPress will use the index template to show the list of posts.
This also means that any content you might have entereed into the ‘posts page’ will not get shown.
Make sure to add any content before you set the page as the ‘posts page’; otherwise the editor will not be shown, and a warning message will be displayed instead.
To show this content of the ‘posts page’ before the list of posts, you can use following code (to be inserted into functions.php of a child theme, the html structure of the example below is based on Twenty Fourteen):
WordPress allows you to set a ‘category base’ to show in the permalinks of the category archive pages – you can set this under ‘dashboard – settings – permalinks – category base’.
However, the latest default theme Twenty Fifteen is using a function to generate the Category Archive page title – get_the_archive_title() – which simply output ‘Category’ and does not reflect on the ‘category base’ entry.
Luckily, the function applies a filter which we can use to influence the Category Archive page title.