He is able to exit Hyperspace between the Starkiller’s shields and the planet’s crust, a feat lifted from an early script of TPM where only Anakin’s Force ability was able to manage such a thing.
And no, the Force has always consistently been something that anyone can learn, but that some have a natural affinity for. That hasn’t changed.
Then why did Rey just suddenly get her powers, without working for anything? It’s explained very clearly in TLJ as the “darkness rises, and light to meet it”. Since Kylo trained for many years to become a dark lord, Rey must get all those powers within a couple days to restore balance (probably because Luke shut himself out).
Because she has that natural affinity for it I mentioned. That and the fact that she took Force knowledge from Kylo when he searched her mind, something which is unfortunately not made very clear in the film.
All the ending shows is one boy suddenly getting force powers.
There is absolutely nothing to indicate it was sudden. In fact, the way he casually uses the Force seems to indicate he’s been using it for a long time.