For a logical view:
Imagine the Shadowfell, the World, and the Feywild as part of 1 apartment building.
The shadowfell is a suite in the basement, the World is on floor 1, and the Feywild is on the top floor.
Being apartments, each suite is generally similiar to the other two. There are a few differences, but the layout of the floorplans are remarkably similar.
Because it's in the basement, the shadowfell is dark, dank, has extra vermin, and has the older, more worn down furniture that others have gotten rid of. Living there is the strange, freaky, morose neighbour who likes to collect spiders and stares at people in a way that creeps them out.
By comparison, the Feywild has the best view, the most sunlight, and lots of windows. The furniture is nice and new, and the place was painted recently. However, the people who live there are kinda stuck up and proud. And their dog is very tempermental, and if you look at it wrong it'll try and bite your face off.
Normally, you move from suite to suite by using the stairs (portals and rituals), but for some odd reason, there are places where the floors and ceilings are weak. With a bit of a push you can climb up through the roof to the higher level, or you can walk over a section of weak floor and crash down into a lower level. These are sections of the planes that are 'weak' and allows for cross-overs to happen. Your neighbours occasionally use these 'weak' areas as well and can show up in your apartment, usually when you don't want them to.
To get to the other planes (Astal Sea and Elemental Chaos) you actually have to leave your apartment and go an entirely different building with its own rules, floorplans, and tenants.