To configure low-level link settings independently of networks, see systemd.link(5).
systemd-networkd will create network devices based on the configuration in systemd.netdev(5) files, respecting the [Match] sections in those files.
systemd-networkd will manage network addresses and routes for any link for which it finds a .network file with an appropriate [Match] section, see systemd.network(5). For those links, it will flush existing network addresses and routes when bringing up the device. Any links not matched by one of the .network files will be ignored. It is also possible to explicitly tell systemd-networkd to ignore a link by using Unmanaged=yes option, see systemd.network(5).
When systemd-networkd exits, it generally leaves existing network devices and configuration intact. This makes it possible to transition from the initramfs and to restart the service without breaking connectivity. This also means that when configuration is updated and systemd-networkd is restarted, netdev interfaces for which configuration was removed will not be dropped, and may need to be cleaned up manually.