NetworkManager [1,2] is a service that runs in the background on your system and allows you to easily set up network connections. It is especially useful if you need to configure a wireless connection, VPN etc. NetworkManager is enabled by default. In graphical interface, there is an icon (or menu) where nm-connection-editor could be used for configuration.
In past versions of CentOS, network devices were configured by network script and by command ifup and ifdown. Since CentOS 8 this method is deprecated and NetworkManager is preferred.
1. Graphical User Interface
In GUI look for icon next to clock (usually at right corner of your screen in notification area). If you left-click on this icon, you will see a list of possible Wireless networks to connect to. If you right-click, you could enable/disable network, show information about network or edit network configuration.
2. Command line
Status of NetworkManager service could be checked by:
systemctl status NetworkManager.service
The service could be started, stopped, enabled or disabled like other services by systemctl command.
3. Configuration files
Configuration files are located in files at /etc/sysconfig/network-scripts/ifcfg-<name>
 http://fedoraproject.org/wiki/Tools/NetworkManager (some gnome- and kde-specific info)