* If you are using Xorg, you put environment variables in ~/.xprofile
* If you are using Xorg on Debian, you put the environment variables in ~/.xsessionrc
* If you are using Wayland, you make a file in ~/.config/environment.d/ and set you configuration variables there.
There is a similar environment variable for Steam on Linux and I would argue that it should be an application setting but for whatever reason it isn't.
Both of these are hacks around how Xorg (doesn't) handle fractional scaling.
I understand that, but your window server is just another application. On Windows I wouldn't consider scale factor being an env var, and indeed it isn't set as such -- rather that value is set in the registry.
Maybe that's the answer -- Windows has a defacto method of setting system-wide env vars that every application inherits within a given personality but Linux/BSD does not.
I can live with that answer :-)
Not the window server.
QT is a library that applications can use to render their GUIs, so there are many instances of QT in all of the apps that use it.
This is not in any way Wayland specific, and arguably should be the default choice for setting env vars, as it should apply to all user sessions regardless of what type they are or what shell is used.