In the comics, Scarlet Witch used her reality manipulating powers to create her twin sons, which appears to be what happened in WandaVision.
Tommy aka Thomas Shepherd grows up to be a Marvel superhero called Speed. Just like his uncle Quicksilver, he is a mutant who can run unbelievably fast.
Meanwhile, Tommy’s brother Billy aka William Kaplan-Altman becomes a Young Avenger called Wiccan.
Just like his mother Scarlet Witch, Wiccan has incredibly magical abilities, including reality warping.