Screenrant does a fantastic job documenting the what and why of the Force Dyad in the Star Wars universe:
The Force Dyad appears to be an entirely natural phenomenon. According to Charles Soule's tie-in comic The Rise of Kylo Ren, it first manifested when Ben Solo began to surrender to the dark side. He was locked in a duel with the leader of the Knights of Ren, and abandoned himself to the darkness for the first time; far away, on the desert planet of Jakku, a young desert rat named Rey suddenly felt cold. It's clear, then, that the Dyad was created by the express will of the Force itself, rather than from intervention by any third party.
The Star Wars: The Rise of Skywalker Visual Dictionary stresses the Dyad can be cultivated. That happened years later, when Kylo Ren captured Rey in Star Wars: The Force Awakens, and attempted to probe her mind. This fits perfectly with Jason Fry's novelization of Star Wars: The Last Jedi, in which he explained this was how Rey learned so much about the Force so fast.
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It's telling that, immediately after the Dyad had been cultivated, Rey was able to use a Jedi mind trick to escape her cell on Starkiller Base; shortly after, she was able to match Kylo Ren in a duel.
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The morality of Star Wars is usually quite simple, with a strong dichotomy between light and dark. The Jedi are seen as servants of the light side of the Force, while the Sith are of the dark. But it's increasingly becoming clear this is a perversion of the original Jedi teachings; Star Wars: The Last Jedi featured a mural showing the Prime Jedi, and it showed a teacher whose philosophy was very different. Light and dark were given equal prominence in the mural, with the Prime Jedi representing the darkness in the light, and the light in the darkness. This was supported by Cavan Scott's audiobook Dooku: Jedi Lost, which revealed the ancient Jedi treated balance as a separate aspect of the Force, like light and dark.
Rey and Ben Solo both know the light and dark sides of the Force. However hard Rey attempts to commit to the light, there is still anger and pain bubbling up within her - explosively so, when she unleashed Force Lightning in Star Wars: The Rise of Skywalker. Ben Solo, meanwhile, has never been able to quiet the siren song of the light side of the Force. Palpatine intended Han Solo's murder to push Kylo to the dark side beyond the point of no return, but Han sabotaged it by his willingness to die for his son, unwittingly ensuring Ben never truly committed to the dark side. Like the Prime Jedi, both Rey and Ben are the light in the darkness, and the darkness in the light. The more they grew in the Force, the stronger their Dyad bond became.
Keep reading the article for more details.
Here's what is great about these new details: the movie(s) never fully developed this idea. Throughout the final trilogy (that focused on Rey), fans were left mouth agape when Rey would flex Jedi skills. When Rey was a captive on Starkiller Base and was able to mind trick a First Order Stormtrooper (ably played by none other than James Bond himself, Daniel Craig), I was left thinking that it took Luke Skywalker several years to hone that skill.
With these additional details, we now can more clearly understand why Rey was so suddenly able to use the Force with the skill of a full-fledged Jedi (or Sith). It would have been nice to have some of this explained in "The Force Awakens" - but at least we're getting it now.