If one were to pointlessly dig too deep into that episode; Serena re-appears with a new tighter shirt that tells she has tits now and her skirt is the shortest it has ever been after spending all of XY(Z) prioritizing hiding her panties despite facing injury or death from a fall (and those skirts were longer).
Ash went from looking like a teen-ish version of himself to a goofy ass mofo who's sole purpose in life is just play with pokemon (since he's a audience surrogate for 6-10 yr olds). Serena is moving on (and will get some BBC) while Ash is still a kid (you're now hearing Iris insult Ash). Now Tamagoro Ash, that boy is a chad.