As seen on the Disney+ series “WandaVision,” Monica’s powers begin to manifest after she comes into direct contact with the borders of the Hex — Wanda Maximoff’s (Elizabeth Olsen) energy field around the town of Westview, New Jersey. Though said series and “Doctor Strange in the Multiverse of Madness” expand her lore to include more mystical elements, the mid-credits scene of “Captain America: The Winter Soldier” reveals that the Scarlet Witch’s powers are awakened by the Mind Stone (then housed in Loki’s scepter). So in the same way a light-speed engine powered by the Space Stone created Captain Marvel, a hex powered by the Mind Stone gives Monica her powers.
It’s also interesting to note both that Monica’s powers manifest with a blue coloring reminiscent of the scepter and that she possesses phasing capabilities — capabilities that are also possessed by Vision (Paul Bettany), another being whose powers come from the Mind Stone. Though Monica’s powers could (and likely will) be expanded in terms of lore just like Wanda’s (possibly even to include elements of her mutant origins), validating her connection to the Mind Stone would go a long way toward deepening her bond with Carol — and theoretically, with Kamala.