The planes of existence are not isolated realms, separate and discreet. Rather, they overlap and mingle, with blurry and ever-shifting boundaries. In fact, the realm of mortals is itself not even a proper plane, but the area of overlap between all four "true" planes.
Welcome to Nexus.
The four planes are homes of creatures and cultures that are alien to the mortals inhabiting the material world. Even so, the mixing of planes results in a variety of planar influences, leaving the echoes of the planes visible in daily life. For example, every elf you meet either was born in a region with a heavy fey influence, or has an ancestor who was. All of the mortal peoples of Nexus bear some mix of planar influence, though the exact makeup is not always clear.
Meanwhile, the influence of the planes is uneven and inconsistent. A haunted forest may be the result of an increased presence of the plane of shadow, and a surge of influence from the elemental plane can cause catastrophic natural disasters.