Builder.kt

class Product {
    var partA: String = ""
    var partB: String = ""
    var partC: String = ""
}

interface Builder {
    fun buildPartA(part: String)
    fun buildPartB(part: String)
    fun buildPartC(part: String)
    fun getResult(): Product
}

class ConcreteBuilder : Builder {
    private val product = Product()

    override fun buildPartA(part: String) {
        product.partA = part
    }

    override fun buildPartB(part: String) {
        product.partB = part
    }

    override fun buildPartC(part: String) {
        product.partC = part
    }

    override fun getResult(): Product {
        return product
    }
}

class Director {
    private lateinit var builder: Builder

    fun setBuilder(builder: Builder) {
        this.builder = builder
    }

    fun buildMinimalViableProduct() {
        builder.buildPartA("part A")
    }

    fun buildFullFeaturedProduct() {
        builder.buildPartA("part A")
        builder.buildPartB("part B")
        builder.buildPartC("part C")
    }
}