It’s possible for those in the Physical World to access the magic that’s housed in the Spirit World. Many creatures can access the power of the Spirit World innately, similar to sorcerers. Others beings who aren’t innately magical can learn to access it’s power as well, by studying and learning how to manipulate the magic that seeps into the world naturally and focusing it. Of course many creatures are funneled magic through other beings who have an innate magical connection to the Spirit World, commonly demons, fey, or celestials. Similarly gods (through their Saints) often funnel their power to mortals who they deem deserving.