In the D&D 5e group I'm playing in we're currently wondering how much our characters (all level 7) would know about the planes in the Forgotten Realms campaign setting (we're playing SKT, though that's otherwise not relevant to the question).
The knowledge in question is only that about the basics of the Great Wheel Cosmology, as described in the PHB in "Appendix C: The Planes of Existence", pages 300-303.
Detailed information about a certain plane, planar creatures or planar politics (i.e. the blood war) obviously requires either a knowledge check or a backstory reason why the character would know this particular kind of information (i.e., a Githzerai character sent from his home in Limbo to fight Mindflayers on the material plane would obviously know about Limbo and its workings).
Chapter 7 of the PHB, "Using Ability Scores", also specifies that the Arcana skill is meant for checks concerning the planes (see quote below), but not whether you need it to know that planes are a thing.
Arcana. Your Intelligence (Arcana) check measures your ability to recall lore about [...] the planes of existence, and the inhabitants of those planes.
That being said, for the purpose of this question, assume that there are 3 different types of people:
- people with proficiency in the Arcana skill
- spellcasters without proficiency in the Arcana skill
- non-spellcasters without proficiency in the Arcana skill
- run-of-the-mill commoners
Which of the following facts about the Great Wheel cosmology would be known to average representatives of these 4 types?
- planes of existence are a thing / there's more than just the material plane
- the overall "layout" / names of the planes, as specified in the PHB on page 303
- the distinction between demi-, transitive, inner and outer planes and how they differ from each other
- methods of planar travel
Clearly, spellcasters - even without Arcana proficiency - have to have at least basic knowledge, otherwise, it wouldn't make much sense to have access to spells such as Banishment (lvl 4), Contact Other Plane (lvl 5) or Plane Shift (lvl 7). Or would you be unable to cast these spells if you don't have knowledge about planes?
I'm primarily interested in any guidance given in official sources, whether they're rulebooks, novels, developer tweets or something else. Since my group is playing 5e, sources for that edition would be preferable, but ultimately, sources for any edition are fine.