[Q] General configuration, subdomain, multiple web services

Hello!
I'm a beginner in setting up a webserver and I'm struggling with the different configuration files.
My first question is:
What is the difference of a config-file that is located in /etc/nginx/conf.d compared to a config-file in /etc/nginx/sites-enabled?

Second question:
What's the best practice for setting up multiple "web services" on a single webserver?
My intention is to run
- Owncloud
- Piwigo
- Adminer
and I assume the configuration of a subdomain, e.g. cloud.domain.local, should work best.

Any recommendation and best practice is highly appreciated.

THX