The Planescape campaign setting is a big place. Not only is the Prime Material Plane basically infinite, but so too are the Outer Planes. If you're looking to get a primer on what the deal is with the Outer Planes, then this is a good place to start. I hope you enjoy lots of made up words, details about fictional worlds, and pictures of fantasy landscapes.

0:00 The Types of Planes
2:27 An Overview of the Outer Planes
8:07 The Outer Planes
8:58 Elysium
9:50 Bytopia
10:40 Mount Celestia
12:37 Arcadia
14:39 A Bit About Modrons
15:46 Mechanus
16:43 Acheron
18:01 Baator
20:10 The Blood War
21:11 Gehenna
22:16 The Gray Waste
24:31 Carceri
25:44 The Abyss
27:12 Pandemonium
28:08 Limbo
29:03 Ysgard
30:26 Arborea
31:35 The Beastlands
32:30 The Outlands
34:37 End

A few sources:
Inner Planes: https://forgottenrealms.fandom.com/wi...
Outer Planes: https://forgottenrealms.fandom.com/wi...
https://en.wikipedia.org/wiki/Outer_P...
Planescape campaign setting book: https://www.yumpu.com/en/document/rea...