player3.kt

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            ComposeTubeTheme {
                var source by remember {
                    mutableStateOf(
                        "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4"
                    )
                } 
                
                Column {
                    VideoPlayer(source)
                    Button(onClick = {
                        // Elephant Dream by Blender Foundation
                        source = "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ElephantsDream.mp4"
                    }) {
                        Text("Another Video")
                    }
                }
            }
        }
    }
}

what is state, is it a function?

Ooops, this is very outdated. state was removed from compose a while ago. The replacement is

remember { mutableStateOf() }

I'm going to edit the gist as well.