There are many models for the arrangement of the Outer and Inner Planes in relation to the Material Plane, each of them different and all of them at least partially true.
At its most basic level the Material Plane exists at the centre of the arrangement of the planes, around that and at the same time existing in the same space are the infinite Inner Planes that provide energy and substance to the Material Plane.
The Inner Planes are actually a single plane that is so vast as to be beyond infinite. The different energies (Earth, Air, Fire, Water) that make up this Inner Plane coalesce into enormous sections that are incorrectly thought to be separate planes in their own right. These energy sections constantly move and change over the course of thousands of millennia, and where the different energies meet they create new energies that are often termed Quasi Elemental Planes of Steam, Lightning, Lava, etc. Occasionally great portions of a section will be calved off and float like an island of energy into a different energy and persist for many millions of years before ultimately being consumed. The Inner Planes are every changing and unfathomably vast, and at their centre lies the great vortex of the last layer of the Abyss that connects all the way to the Outer Plane.
Around the outside of the Material Plane are the Outer Planes, many believe the Outer Planes are arranged in blocks around the outside of the sphere that is the conjunction of Material and Inner Planes, but in reality each Outer Plane is a pattern of constantly morphing bridges that connect to other Outer Planes and make the Outer Planes resemble an overlapping layer of planes (each nearly infinite in size) with a spider web of bridges and connections to other planes.
Throughout the entire planar cosmology are the Positive Energy Plane and the Negative Energy Plane that give life and death to everything in the Multiverse. There planes are so powerful that they consume all matter, and so are separated from the Material, Inner, and Outer Planes, occupying the same relative planar space but isolated from it by a supposedly impenetrable planar boundary. The leakage from these energy planes is what gives life to all beings in the Multiverse (positive energy being the basic element of a soul), or allows undead creatures to exist and propagate.
Lastly there exist the Transitive Planes that act as boundaries between the Material, Inner, Outer, and Energy Planes, limiting the exposure and connection between the sometimes antagonistic energies and elements that exist. The Ethereal Plane borders the Material and the Inner Planes. The Astral Plane borders the Material and the Outer Planes. The Faeree Plane borders the Material and Positive Energy Plane. The Shadow Plane borders the Material and Negative Energy Plane.
Limited connections between the various planes are possible using Gates and Portals, but normally connections are only made between planes of the same type (Outer Planes or Inner Planes) or between planes and the Material Plane.