All Systemd services can be divided into two categories. The first category is system services which are started by a superuser. You have to log in as a root user or use the sudo utility to manage these services. The second category is the services which can be started from a specific by and this user has full control over them without sudo.
You can find files for the system services in these directories:
Files for user services can be found in these directories:
Note that services which are provided by packages are usually located in the /usr/lib or /lib subdirectories. Modified unit files that override default unit properties are located in /etc. You can read more details about it all in the article Using Systemd Services in Linux.