How do you put new content into templates? Is website content uploaded through design manager?
Not quite.
Design manager is indeed where you build and manage your templates. These can be blank, although they are often created with a page in mind and so may have content in them.
To add content to a page you first need to create it in either the ‘Websites’ or ‘Landing Pages’ part of the marketing tool. There you can pick a template on which to base your page and then populate or customise the content.
Hope that helps.