What defines Shademar appearance.

[Solarserpent]

In Scadrial books Shadesmar is one of Mists and in Roshar it is full of Spren and countless connections to the physical world rolled up in little spheres. Are these difference defined by the collectivve minds of the people or is it more defined by the Investiture/Shard in charge (Roshar was originally Adonalsium's world so the spheres and most of the spren are probably his and not specifically Honor, Cultivation, or Odium's). Is the only constant to Shadesmar the beyond. Is the inverse landscape a constant? How much can a Shard change the landscape. Brandon has said that Sel is a dangerous place in the Cognitive realm because of the Shattered shards power residing mainly in the cognitive realm so how far can the cognitive realm be twisted by Investiture. Perhaps there is a splinter of Ambition causing the cognitive realm to be twisted on threnody.

[RShara she/her]

It has to do with how the sapient beings think of the area, plus influence from how investiture manifests.  So kind of both

The Beyond is something else, separate from the Spiritual, Cognitive and Physical Realms, and is an area that the Shards can't touch or see into.  Brandon's deliberately left it vague on whether the Beyond actually exists or not.

The inverse landscape seems constant.  My theory is that people see land as having a bunch of separate objects on it--dirt, rocks, roads, buildings, etc, so it's a bunch of beads, or mist or whatever objects manifest as.  Whereas people see the ocean or a river or a lake as a single object, so it's solid.

I don't think the Shards can affect the landscape very much, other than, you know, being able to rearrange the Physical aspect, thus changing the Cognitive Aspect.  The collective thought of masses of populations over long periods of time has more of an effect.

Sel is dangerous because all of the power of 2 somewhat opposing Shards is crammed in there.  So they're like a plasma storm--fighting and surging and spinning, random arcs of energy flashing around, etc.  I doubt they're affecting the landscape, other than making it perilous.

I agree that Ambition and Odium's fight with Ambition has affected Threnody and its investiture.  How, that's still a big question!

[Weltall]

All good points. I'll just add that Brandon has stated that the land/water inversion isn't universal across the Cognitive Realm. We just haven't seen any examples to the contrary yet.