explore whats happening here in swift?

sample_code.swift

class Car {
    var plate: String
    var driver: Driver?
    
    init(plate: String) {
        self.plate = plate
    }
}

class Driver {
    let name: String
    var cars: [Car] = []

    init(name: String) {
        self.name = name
    }
}


var myCar: Car? = Car(plate: "ABC123")
var myDriver: Driver? = Driver(name: "Neo")

myCar?.driver = myDriver

if let car = myCar {
    myDriver?.cars.append(car)
}


myCar = nil 
myDriver = nil