For example, I have a blog and have recently added an extra page. If I post things on this extra page, will those following my main blog have it come up in their newsfeed? Or do they have to follow the page seperately?
If you are referring to custom pages: No, followers don't get notified if you create or edit a custom page. You'd usually use such pages for "static" content, like "About me" or "Contact".
If you are referring to additional blogs: No, followers of your main blog don't get notified if you create or update a secondary blog. They'd need to follow this blog separately.