Putting something below a LazyColumn in Jetpack Compose

You can apply the weight modifier to the lazyColumn:

Column(Modifier.fillMaxSize()) {
    LazyColumn(Modifier.weight(1f)) {
        items(50) { i ->
            Text("Row $i", Modifier.fillMaxWidth().padding(8.dp))
        }
    }
    Button(onClick = { println("hi") }) {
        Text("Hello")
    }
}

This is also explained in the Jetpack Compose basics codelab