I've created a post for this over in the /r/nginx Reddit community:
https://www.reddit.com/r/nginx/comments/bq6g77/welcome_to_nginx_and_apache2_ubuntu_default_page/

How come I'm getting the "Welcome to nginx!" and "Apache2 Ubuntu Default Page" default pages? Where do I find these "default" pages in Nginx? The default Apache page is very odd for ownCloud. Its running Apache for its SSL, so the default page for it could be on the ownCloud VM. Meanwhile Discourse does not have Nginx installed on it at all.

Can I redirect from one link to another with Nginx? For example, could I redirect from https://oc.myreserveddns.com to https://oc.myreserveddns.com/owncloud/index.php/login?

If you need more details, please let me know.