XQuartz and GTK apps on MacOS are the exception. A bigger problem with regards to inconsistent UI/UX stems from Electron apps and apps like Firefox which implement its own context menus, for example, and don't interact with the OS with things like Applescript, which Chromium browsers and Safari do. Still, there's a lot of applications that do respect the OS paradigms for design and user experience. Interfacing with applications is fairly consistent, though this has been degrading for some time with the rise of Electron.
On linux, there is no paradigm to stray from and there's no consistent design language or UX because of the fractured nature of its ecosystem. There are incredibly well made applications for KDE, GTK, and Electron, all with their own ideas of what UX should be and what UI it should look like.
