class A(val name: String, val country: String)
class B(val name: String, val age: Int)
class C(val name: String, val age: Int, val country: String)

List<A>

List<B>

List<C>

C

A and B

a.name == b.name

val a = listOf<A>()
val b = listOf<B>()

val c = a.zip(b)
    .filter { (a, b) -> a.name == b.name}
    .map { (a, b) -> C(a.name, b.age, a.country) }

groupBy

fun toC(a: List<A>, b: List<B>): List<C> {
    val names = a.map(A::name).union(b.map(B::name))
    val aa = a.filter { names.contains(it.name) }.sortedBy(A::name)
    val bb = b.filter { names.contains(it.name) }.sortedBy(B::name)
    
    return aa.zip(bb).map { (a, b) -> C(a.name, b.age, a.country) }
}

val listA = listOf(1, 2, 3, 4, 5)
        val listB = listOf(1, 2, 6, 7, 5)
        val listC = mutableListOf<Int>()
        listA.fold(listB) { acc, i -> if (acc.contains(i)) listC.add(i); acc }
print(listC) // [1, 2, 5]

fun toC2(a: List<A>, b: List<B>): List<C> {
    val aa = a.associateBy(A::name)
    val bb = b.associateBy(B::name)
    
    return aa.keys.union(bb.keys).map {
        C(it, bb[it]!!.age, aa[it]!!.country)
    }
}

!!

listA.mapNotNull { a -> 
    listB.firstOrNull { b -> a.name == b.name }
            ?.let { b -> C(a. name, b. age, a.country) }
}