Kotlin Stack Implementation

Stack.kt

class Stack<T>{
    val elements: MutableList<T> = mutableListOf()
    fun isEmpty() = elements.isEmpty()
    fun count() = elements.size
    fun push(item: T) = elements.add(item)
    fun pop() : T? {
        val item = elements.lastOrNull()
        if (!isEmpty()){
            elements.removeAt(elements.size -1)
        }
        return item
    }
    fun peek() : T? = elements.lastOrNull()

    override fun toString(): String = elements.toString()
}

fun <T> Stack<T>.push(items: Collection<T>) = items.forEach { this.push(it) }

why is push written outside of the class decleration

It's an extension function

Official kotlin doc, this might help.
https://play.kotlinlang.org/byExample/01_introduction/06_Generics