During the Claw Island arc, we entered the Dream to see a landscape of Orr. It was not really Orr, of course; just the Dream's representation of it. Likewise, it was also not a memory of one seeing Orr, though we are told the Dream is made up of memory.
This is the primary way we have encountered the Dream and Dream-like things: for example, Mordremoth's mind, or Caladbolg's visions. Yet we believe the Dream of Dreams, the pre-life Dream of the sylvari, is an experience almost solely of memory: the memories of sylvari that have awakened.
How much agency does one have in these memory-visions? I think not much. Sylvari are said to desire experiences to differentiate themselves from other sylvari who have seen the same or similar visions.
I believe the pre-life Dream is a mixture of dreamscapes and visions. The sylvari tutorial instance takes place inside a real dreamscape - unlike the landscapes of Orr and of Mordremoth's mind, the real dreamscape is a little more random, like real dreams; various things all put together. But not a direct vision or memory, just a dreamscape.
Ventari is in this tutorial dreamscape, as is a quaggan and a norn. Quaggan did not populate the continent of Tyria while Ventari was alive; they were still in deep waters. I think these dreamscapes are formed of memories; that is, they incorporate elements of many different memories (as real dreams incorporate the day's experiences), but it is not a vision.
What is a vision? It is a memory of another: an experience they had, seen through their eyes, and a Dreaming sylvari may even experience their emotions. These are... more directed than dreamscapes. A vision is given to a sylvari by the Dream, or the Pale Tree guides them into it to learn something. Thus, these visions may be a combination of two or more memories: but they will be memories of very similar things.
For example, we know that before the Personal Story, only two sylvari had ever seen Zhaitan: Trahearne and Caithe. A sapling could very easily get both of these at the same time; perhaps they would experience the emotions of both, or see the dragon from two different angles at once, or the continuity of the ground below would be awkwardly messed up. But the point and meaning of the vision - Zhaitan - would remain.
A sylvari may experience a memory of a sylvari casting magic; then in the dreamscape, they can practice it on their own.
I imagine most dreamscapes are like the tutorial one; more put together than multiple visions of the same thing, with contradictions only really notable if you know history (as in the case of Ventari and quaggans in the same room) or the like.
There is a question I have had for a long time: when you meet Caithe, who says "can you hear me where you are, in the Dream?" your character responds, "in the Dream? What does that mean?"
Your character knows what the Dream is. They was just talking about it earlier. So why this question? I believe the reason is that your sylvari doesn't perceive there to be an 'outside the Dream.' They has never questioned where the memory-visions come from.
Life, to an un-awakened sapling, is just dreamscapes and memories. That is how the world works. They are aware of the Pale Tree (Anet blog posts have said the Pale Tree makes sure each sapling knows they is loved) but don't quite understand where or what it is, exactly, except that she's a person who loves them and will take care of them.