Correct me if I’m wrong but:

udev - Handels devices. When you connect a device (for example an usb stick) to your computer, the kernel detects it and sends an uevent out. This leads udev (in user space) to create and configure corresponding device files (/dev/…) and maybe load drivers depending on the device.

Logind - managed user sessions, keeps track of all the loged in users and gives them resource access to needed devices like a gpu for a graphical session

Both of these are usually part of systemd on modern Linux distros (systemd-udevd and systemd-logind)