addAll
Adds all elements of the given elements collection to this MutableCollection.
@IgnorableReturnValuefun <T> MutableCollection<in T>.addAll(elements: Iterable<T>): Boolean(source)
val numbers = mutableListOf(1, 2, 3)
val moreNumbers = listOf(4, 5, 6)
// add all elements from `moreNumbers` to `numbers`
numbers.addAll(moreNumbers)
// `numbers` now contains: [1, 2, 3, 4, 5, 6]