The Ethereal Plane is a misty, fog-bound dimension that is sometimes described as a great ocean. Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.
DMG, p. 48:
From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.
First question: does the Border Ethereal also overlap the Feywild and Shadowfell?
Now, if an ethereal creature, such as a phase spider or a ghost, for some reason travels to one of the Outer Planes:
Can these creatures travel to the Outer Planes, or are they not living beings capable of surviving there? Maybe it depends on the specific creature? DM's decision?
I guess abilities like "shifting from the Material Plane to the Ethereal Plane, or vice versa" are not possible in the Outer Planes, right?
Yes as described in the DMG the the Border Ethereal connect to the Shadowfell and as a specific curtain color of the curtain as indicated on the Ethereal Curtains table.
The Border Ethereal, overlap the Material Plane and the Inner Planes, not the Outer Planes.
A ghost,'s Etherealness only let it enters the Ethereal Plane from the Material Plane, or vice versa. A phase spider's Ethereal Jaunt likewise let it shift from the Material Plane to the Ethereal Plane, or vice versa. Neither can use their trait to the Innper Planes or Outer Planes.
Deep Ethereal: Scattered throughout the plane are curtains of vaporous color, and passing through a curtain leads a traveler to a region of the Border Ethereal connected to a specific Inner Plane, the Material Plane, the Feywild, or the Shadowfell. The color of the curtain indicates the plane whose Border Ethereal the curtain conceals; see the Ethereal Curtains table.
PHB, p. 301:
DMG, p. 48:
First question: does the Border Ethereal also overlap the Feywild and Shadowfell?
Now, if an ethereal creature, such as a phase spider or a ghost, for some reason travels to one of the Outer Planes:
Yes as described in the DMG the the Border Ethereal connect to the Shadowfell and as a specific curtain color of the curtain as indicated on the Ethereal Curtains table.
The Border Ethereal, overlap the Material Plane and the Inner Planes, not the Outer Planes.
A ghost,'s Etherealness only let it enters the Ethereal Plane from the Material Plane, or vice versa. A phase spider's Ethereal Jaunt likewise let it shift from the Material Plane to the Ethereal Plane, or vice versa. Neither can use their trait to the Innper Planes or Outer Planes.
Nice! Thank you very much for the quick answer, @Plaguescarred!