At 7:17, why use U and T when assigning the property values, rather than newMonster and monsterIn respectively?

import UIKit

class Monster {
    var lifeForce: Int
    var invincibility: Int
    var position: Int //should be something like a Point, but I made it an Int instead, for simplicity

    init(lifeForce: Int, invincibility: Int, position: Int) {
        self.lifeForce = lifeForce
        self.invincibility = invincibility
        self.position = position
    }
}

class Goblin: Monster {
    var smell: Int

    init(lifeForce: Int, invincibility: Int, position: Int, smell: Int) {
        self.smell = smell
        super.init(lifeForce: lifeForce, invincibility: invincibility, position: position)
    }
}

class Troll: Monster {
    var jewels: Int

    init(lifeForce: Int, invincibility: Int, position: Int, jewels: Int) {
        self.jewels = jewels
        super.init(lifeForce: lifeForce, invincibility: invincibility, position: position)
    }
}

class Vampire: Monster {
    var numHollowTeeth: Int

    init(lifeForce: Int, invincibility: Int, position: Int, numHollowTeeth: Int) {
        self.numHollowTeeth = numHollowTeeth
        super.init(lifeForce: lifeForce, invincibility: invincibility, position: position)
    }
}

func randomMonsterGenerator() -> Monster {
    let randNum = Int(arc4random_uniform(3) + 1)

    switch randNum {  //the values should be random too, but for simplicity ...
    case 1:
        return Vampire(lifeForce: 2, invincibility: 3, position: 5, numHollowTeeth: 6)
    case 2:
        return Goblin(lifeForce: 2, invincibility: 3, position: 5, smell: 26)
    case 3:
        return Troll(lifeForce: 6, invincibility: 5, position: 3, jewels: 12)
    default:
        return Monster(lifeForce: 2, invincibility: 3, position: 5)
    }

}

func magicSpellNonGeneric(monsterIn: Monster) -> Monster {

    let newMonster = randomMonsterGenerator()

    newMonster.lifeForce = monsterIn.lifeForce
    newMonster.invincibility = monsterIn.invincibility
    newMonster.position = monsterIn.position

    return newMonster
}

let m1 = Goblin(lifeForce: 5, invincibility: 6, position: 23, smell: 25)
magicSpellNonGeneric(m1)

func magicSpell<T, U>(monsterIn: T) -> U {

    var newMonster: U = randomMonsterGenerator()
    //Cannot convert value of type 'Monster' to specified type 'U'

    U.lifeForce = T.lifeForce
    //Type 'U' has no member 'lifeForce'

    U.invincibility = T.invincibility
    //Type 'U' has no member 'invincibility'

    U.position = T.position
    //Type 'U' has no member 'position'

    //remove monsterIn
    return U
    //Cannot convert return expression of type 'U.type' to return type 'U'
}