I was going to explain how Outsider is based on where they're native, not where they go on death, but I would have been mistaking Outsider for Extraplanar...
An outsider is at least partially composed of the essence (but not necessarily the material) of some plane other than the Material Plane. Some creatures start out as some other type and become outsiders when they attain a higher (or lower) state of spiritual existence.
Anyway, I think it is a common thing that Outsiders reform on their home plane (probably not The Astral Plane, it's a weird place). I've heard that's true for demons and devils at least, and that it doesn't occur if they're killed while home - IIRC they dissipate into the plane, truly dead without the ninth level True Resurrection. In 5e it looks like they reform instantly, in 3.5 I think it took hundreds of years (666 for devils, 999 for demons?).
Mortals just have their souls snatched up by gods (or possibly a plane) on death... Or their souls just stick around the Material/Etherial planes as ghosts (or Shadows maybe?). The Etherial plane is kinda unique in that it maps 1-to-1 with the Material, and is "close", which is why it's relatively easy to affect both at once.
Weirdly,
Elementals are not
Outsiders, just
Explanar creatures... who specifically can't be resurrected normally. So practically speaking, Outsiders. I'm not sure why 3.5 did it that way.
Any creature that's away from its home plane is considered Extraplanar, heh, including normal adventurers. I assume that's used for banishment spells... yeah it is, makes sense. Can't banish a devil when you're in Hell!
Edit: I'm rambling all over the place as I keep checking the references, but:
...Does that make mortals outsiders?
Nah, because mortals have a body and also a soul. Outsiders (and Elementals!), their body IS their soul. Monks at level 20 also fuse their body and soul, which does... little, since they still get to resurrect normally as a special case.