Not really because then you would need to implement composition from different processes yourself, instead many are using Qt Wayland https://wiki.qt.io/QtWayland to implement it. One of those implementations is https://doc.qt.io/QtApplicationManager/ which uses wayland to compose the UI from 3rd party applications, which run in a sandbox, inside of the System UI.

I see, on my naivety regarding your domain, I thought there would be just some kind of UI process/thread doing all the GUI stuff on the Infotainment UI.

Since I am only familiar with BMW attempts at JavaFX (described at JavaONE) and from what I am aware most embedded GUIs aren't X compatible, although there used to exist TinyX.

So is Qt kind of winning all the new GUI projects in that domain?

It's a bit weird, many try HTML5, some try Android, many do Qt but some run something with help of Weston. It's difficult to say for me who is winning because I'm not involved in enough different projects I feel.