17 "Agatha All Along" Easter Eggs From Episode 3 That You Might've Missed The First Time Around

If you want a little explanation, in the comics, after Billy and Tommy Maximoff disappeared, Mephisto (more on him later) reclaimed their souls, which ultimately destroyed the demon with Wanda's help. Eventually, Billy's soul was reborn in Billy Kaplan, the son of Jeff and Rebecca Kaplan. 

Growing up, Billy's favorite Avenger was the Scarlet Witch — like, same — and he ultimately joins the Young Avengers as Wiccan. He has similar powers as Wanda except while hers manifest as red magic, his manifest as blue magic.

Eventually, Billy learns of his true heritage, aka being the reincarnated son of Scarlet Witch and Vision. And, yes, Tommy is also reincarnated as Tommy Shepherd, aka Speed.