```mutableListOf<Something>().apply { ...

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[Czar]

mutableListOf<Something>().apply {
        add(item1)
        addAll(subList)
}.toList()

This creates one list and adds one element to it, then adds all elements from second list, then creates a new immutable list from it.
listOf(item) + sublist creates an immutable list from one element, then creates second list with this item and items from the third list.