why would I need some androidx library on my linux desktop? And why is the text only centered horizontally ?

To center inside that height modifier, use

.fillMaxHeight().wrapContentHeight()

To answer your first question, Compose, and many adjacent libraries, were originally created for Android and so use the

androidx

package name. They then became not Android specific anymore, but there was no reason to change the package names. I also work on a desktop app and it’s full of “android” in imports, even though it has nothing to do with Android. It’s nothing to worry about though, it’s just a historical artifact of the naming.