I don’t know what it’s called so, searching on how to change it is a problem…
Compared to, for example Debian XFCE, the buttons on popup dialogs show in different order.
For example, on Debian, “Ok, Cancel”, on Manjaro “Cancel, Ok”.
Is this a user setting? Where can I find it?
In this case it is xfce on Debian and Plasma on Manjaro.
Apples vs. Oranges
Same colour yet different texture and taste.
How a dialog button is placed is decided by the designer of the application providing the dialog - this is not decided by the operating system.
Although there is design guidelines - they may be different between tool kits - GTK vs. Qt - and - highly probable - 
the mood of the designer …
Perhaps this still applies:
https://bbs.archlinux.org/viewtopic.php?id=239698
You can reverse the order in GTK with gtk-alternative-button-order = 1 in ~/.gtkrc-2.0 and/or $XDG_CONFIG_HOME/gtk-3.0/settings.ini.
The same app on both PC’s and the designer decided to implement differently based on on display manager?
Not the display manager, no. But definitely the graphics framework,
You didn’t mention anything about any application and we haven’t touched theming yet. We don’t know either if you are mixing desktops - both Xfce and Plasma on the same system - such mixing only creates frustration.
GTK is the toolkit used for GTK based systems Xfce among others and Qt is a toolkit used by Plasma and LXQt.
It may be the chosen theme which switches the layout.
Sorry.
I run the the same app (it does use qt) and the buttons come up in different order. Quite a problem when muscle memory expects the Ok where the Delete is.
Absolutely … I totally get it - one thing is to make sure you are using the same theme …
I deal with my users muscle memory every day … CRM web application …
Which app are we talking about?
Also are you sure the “app” is the same version on both? Sometimes things get changed from an older version to newer.
Pretty sure they are the same build.
Both from github, on Manjaro it is AUR freecad-git. That produces the same git build id when I build from git on Debian. It has been this way for as long as I remember.
Make sure the GTK theme is the same on both.
Pretty sure Freecad is qt5, so I doubt gtk theme makes a difference.
The default dialog button order is dictated by the desktop environment. KDE is the same Ok/Cancel order as Windows, GNOME and other GTK-based DE’s like Xfce is Cancel/Ok order like macOS.
I think that using a GTK theme in KDE will only change the button order in GTK apps. Qt apps should follow the DE preference so change it in Xfce I guess.
Or much simpler solution is don’t use two DE’s which have opposite opinions about button order 
Xfce can use qt5ct or kvantum-manager for managing QT theme
qt5ct Interface tab has 4 options for Dialog buttons layout:
Windows, Mac OS X, KDE, GNOME
kvantum-manager Configure Active Theme tab has 6 options for Dialog button layout:
Follow style, KDE layout, Gnome Layout, Windows layout, Mac layout, Android layout