Udev is part of systemd.

It manages the directory /dev

Default system rules for udev

udev rules shipped from packages are in /usr/lib/udev/rules.d/

here we can see what happens what happens when a attached device says it’s a mouse. cat 70-mouse.rules
do not edit this file, it will be overwritten on update

ACTION==“remove”, GOTO=“mouse_end” KERNEL!=“event*”, GOTO=“mouse_end” ENV{ID_INPUT_MOUSE}=="", GOTO=“mouse_end”

#mouse::vp:name::* KERNELS==“input*”, ENV{ID_BUS}==“usb”,
IMPORT{builtin}=“hwdb ‘mouse:$env{ID_BUS}:v$attr{id/vendor}p$attr{id/product}:name:$attr{name}:’”,
GOTO=“mouse_end” KERNELS==“input*”, ENV{ID_BUS}==“bluetooth”,
IMPORT{builtin}=“hwdb ‘mouse:$env{ID_BUS}:v$attr{id/vendor}p$attr{id/product}:name:$attr{name}:’”,
GOTO=“mouse_end” DRIVERS==“psmouse”, SUBSYSTEMS==“serio”,
IMPORT{builtin}=“hwdb ‘mouse:ps2::name:$attr{device/name}:’”,
GOTO=“mouse_end”

LABEL=“mouse_end”

User managed rules

Your own rules can be put in /etc/udev/rules.d/

https://www.linux-community.de/ausgaben/linuxuser/2019/05/unsichtbarer-helfer/3/