The link between the elves and the fey is still mysterious - perhaps the elves are descended from the fey, or are themselves fey, though this is debated. Elven traditions speak of them coming from an ancient 'woodland home', and being expelled from it long ago. Some elves, particularly the Austvagoy in the north, believe that their dead return there and exist as ancestral spirits whom they can contact. This may also be true of fey creatures, although we don't know enough about them to be sure.
The tales say that the First World was created as a draft of the Material Plane, so it contains many similar features - trees, mountains, rivers - a fertile and wild land, as well as dangerous. But it is more fluid, more flexible - time and distance are malleable, geography can adapt to the will of powerful beings. But some mistake was made, perhaps, or whatever being created it - Serpent, or some even greater force - started again for unknown reasons.
While scholars believe their plane must have an Arch of some type, what form it takes or how it is used is unknown.
Gates to the First World from the Material Plane are powerful... artifacts? entities? called the Irminsul. They take the form of massive, ancient trees. The wood is considered very useful for creating magic items or casting powerful spells, and so many have been destroyed over the years. It's very likely that there was one at Seda Grove.