Ok, perspective…don´t cringe now.
It goes like this. No matter where you look in nature or all around you, you can always pull some imaginary straight lines from inside a central point of a feature you´re looking at.
The only way to pull dozens of imaginary lines in your direction that don´t overlap each other is to give them different angles.

The degree of each angle does not matter now. Still, it´s helpful that you make sure you imagine the lines on your left to have the same inclination as the ones you pull on the right, just so that you can have a nice grid and not make a mess right at the foundation of what you´re going to use to grow a scenery out of it.
You must ensure these lines do not overlap naturally as you spread them from a single point to the side. They create that sense of 3D space because they occupy your pic’s ground area.

Then again, raise the horizon level, and you get plenty more ground area to fill in later.

The angle of the pulled imaginary lines from the vanishing point also determines the angle at which the viewer sees your scenery.
If I had placed those pulled lines at a tighter angle to the central one right from underneath the house, the floor would look much more inclined than it is now.

If you don´t get what I´m saying, try to pull some lines with different inclinations to see the illusion effect you get.

In this case, I´ve spread those lines at wider angles across the canvas. So, my ground area became less inclined than the one from the other example above, independently of the fact that this is the version with the horizon placed up.

The angle at which you pull the imaginary lines from each vanishing point determines the illusion of looking down in a balanced way or creates the effect of a really high, straightforward drop.

Experiment with pulling those lines from an object and try to create all sorts of grids yourselves to get a more realistic idea of what I´m trying to explain here.

And, of course, this is the version with many sky areas to fill in.
How do you fill in that area in a realistic way?
Oh, yes, once again…perspective, vanishing points, and some new imaginary grids to place things in. Mostly clouds.

More on this later, at the right time.

Those central points on each feature from where you are pulling those imaginary lines are what´s usually called “vanishing points,” and they´re one of the most important things you need to focus on when you´re creating a landscape, perhaps to illustrate a children book or rendering a simple view.
A complex scenery can end up having dozens of individual vanishing points. Each feature you want to put on the scenery needs those points (and their guidelines) to be correctly placed on a 3D ground while relating well to the other scenery elements you have.

Are you cringing now? It´s not that difficult. 😉

Look at my little house there. This is the most straightforward way to have a landscape. Okay, it´s not that impressive, but even with only the house and the guidelines, you already have a 3D space that people will immediately recognize.

And all this without even adding anything else to the scenery. Anyone looks at this and imagines a house, the ground area, and the sky.

So far, so good. You´ve created a little house on the horizon, pulled those imaginary lines on the ground, and all the 3D effect looks good.
But what happens if you want to add another house on the side? Do you use the same lines you pulled from the central little house?
And why did you pull those lines in the first place ?!
Well, more on this ahead, but for now, you just place another house on the side right on top of the horizon and pull more lines from it.

You start by pulling a straight line right from underneath as you did on the original house and then keep pulling lines to its sides.
This creates another grid that overlaps the original one. Don´t worry, this is what you want, only you don´t know it yet.
Notice those original horizontal lines parallel to the horizon, which make the original green grid? The closer to you, the bigger the spaces between them; as you have those, you don´t need to create more to represent the floor.