Ubuntu 20.04 XDG_SEAT, XDG_VTNR, XDG_SESSION_ID env variables are not set

I set up and installed Ubuntu 20.04 using debootstrap method

  debootstrap --arch=amd64 --variant=buildd focal ./focal_amd64  http://archive.ubuntu.com/ubuntu/ 

then installed all the rest. It runs well except old and new GNOME bugs. I have been using it for ten days. The day before I noticed that it's missing XDG_SESSION_ID. When I dug in I noticed that it's also missing XDG_SEAT, XDG_VTNR, XDG_SESSION_ID variables.
I read that those variables are set by pam modules https://www.freedesktop.org/software/systemd/man/pam_systemd.html

I added session optional pam_loginuid.so and session optional pam_systemd.so to /etc/pam.d/lightdm.

I tried both of the display managers gdm3 and lightdm. They both fail to set environment variables: XDG_SEAT, XDG_VTNR, XDG_SESSION_ID.

XDG_SEAT_PATH is only set in lightdm

I am having a difficulty at debugging it. How can I know which process set those variables which is missing in my system?