What Linux subsystem manages device nodes in /dev? udev does. It is the Linux userspace device manager responsible for creating, removing, and naming device nodes as hardware appears or disappears.
The Linux kernel detects hardware and sends device events to udev, which applies administrator-defined rules. Those rules can assign predictable names, set permissions, create symbolic links, and trigger programs. This is especially useful for hot-plugged devices such as USB drives, keyboards, and network adapters.
udev replaced older approaches including devfs and the hotplug system. A common mix-up is sysfs: sysfs exposes device and kernel-object information, but udev uses those events and attributes to manage the corresponding entries in /dev. On many modern distributions, udev is bundled with systemd, although its device-management role remains distinct from systemd's init and service-management functions.