Is it possible to make a wordpress multisite on domain.com and then every site that I make goes to domain.com/directory/newsite?
no, this is not possible without some major code hacking as far as I can tell. I run a few multisites, and I have some local installations to play around and didn't manage to find a way to do this within the exsiting process-flow, although I would be happy to be proven wrong.
However, there is a way to achieve a similar result with a single-site installation in the root directory and a multisite sub-directory installation in a sub-folder. This would give,
- domain.com as the single site installation in the root folder.
- domain.com/folder/ as the main site of the sub-directory multisite installation which would need to be redirected back to the domain.com.
the downside of this approach is that it would create 2 separate dashboards and 2 separate DBs, although it would be easier to maintain/upgrade on the long-term than a hacked WP multisite installation.
and then I have another question: is it also possible to that every new site has no content, so the whole site is empty?
by this question I am assuming you mean no pages/posts which are created by default on creation.
When a new site is created the wp_insert_site action hook is fired, so you can use that to clear your content with the switch_to_blog function call. I would use wp_delete_post as there is only 2 posts to delete.
if you use wp_cli there is the function wp_site_empty which you could use to the do the same thing, but that is over-kill in my opinion.