printenv – print all or part of environment

X11/XOrg

xprofile

Important: does not work on Debian ! -> .xsessionrc

~/.xprofile

https://wiki.archlinux.org/index.php/Xprofile

Debian + Xorg

~/.xsessionrc

export QT_FONT_DPI=192

https://wiki.debian.org/Xsession

Wayland

Use SystemD…

SystemD

The simplest way to set environment variables for a single user under systemd (version 233 or later), is to create a .conf file in ~/.config/environment.d, containing lines of the form NAME=VAL. See the environment.d documentation for more details.

~/.config/environment.d/*.conf
/etc/environment.d/*.conf
/run/environment.d/*.conf
/usr/lib/environment.d/*.conf
/etc/environment

Source: https://wiki.gnome.org/Initiatives/Wayland/SessionStart