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7.3.2.2. Device Node Creation
Device files are created by the kernel by the devtmpfs filesystem. Any driver that wishes to register a device node will go through the devtmpfs (via the driver core) to do it. When a devtmpfs instance is mounted on /dev, the device node will initially be created with a fixed name, permissions, and owner.
A short time later, the kernel will send a uevent to udevd. Based on the rules specified in the files within the /etc/ udev/rules.d, /lib/udev/rules.d, and /run/udev/rules.d directories, udevd will create additional symlinks to the device node, or change its permissions, owner, or group, or modify the internal udevd database entry (name) for that object.
The rules in these three directories are numbered and all three directories are merged together. If udevd can't find a rule for the device it is creating, it will leave the permissions and ownership at whatever devtmpfs used initially.