[kotlin] val list = mutableListOf(1, 2, 3) + mutableListOf(4, 5)

val list = mutableListOf(1, 2, 3)
println(list) // [1, 2, 3]

list += listOf(4, 5)
println(list) // [1, 2, 3, 4, 5]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/mutable-list-of.html

mutableListOf - Kotlin Programming Language