Help, how to update arraylist in jetpack compose

I’m stuck trying to understand how to get a variable initialised, and later updated, in kotlin to update in Jetpack Compose and cause a composition that uses it to recompose. The variable is an arraylist of bitmaps set up like so (all of this works fine so I have not included the getOutputDirectory() and imageReaderNew() functions):

// set up an empty bitmaps arraylist
// should I be using mutableListOf() here? if so how?
var bitmaps: ArrayList<Bitmap> = ArrayList()

// get the uri of the folder to save images to
val outputDirectory = getOutputDirectory()

// read in a list of images in the images folder
var fileList: ArrayList<File> = imageReaderNew(outputDirectory)

// get a list of bitmaps of the images
bitmaps = getBitmapList(fileList)

When I initialise my composition I do this:

BottomSheetScaffold(
scaffoldState = scaffoldState,
sheetPeekHeight = 0.dp,
sheetContent = {
PhotoBottomSheetContent(
bitmaps = bitmaps,
//should I be using remember here?
//bitmaps = remember { bitmaps },
modifier = Modifier.fillMaxWidth()
)
}
)
{ }

Finally, in my composable I do this:

fun PhotoBottomSheetContent(
bitmaps: List<Bitmap>,
// should I be using mutableListOf() or remember { } here? again if so how?
modifier: Modifier = Modifier
)
{
}