Hello guys.
I have the following following doubt:
I’ve drawn a quarter of a circle and I would like to lift one ending point of this arc vertically (point B). (see the picture)
Why, when I’m lifting point B straight upright, the arc deforms (as in figure 2), while when lifted vertically it should look like in figure 3.

Why should it look like a straight line? If that’s what you want you have to draw a straight line instead of a quarter circle to begin with…
A quarter circle is made of segments. All the vertices move up proportionly so that is what it’s supposed to look like.

Do it like figure 3.
I see what you mean. You need to use Curvishear plugin. I don’t know what the justification is for the way it works simply by moving the point in SU, but it doesn’t serve for most purposes that I’ve encountered. Figure 2 does maintain the shape as an arc and 3 is some sort of curve.

No. Both shapes are arc. But all points in figure 3 are lifted upright in Z direction.
In figure 2 points are moving, also in X and Y directions, so the arc is increasing and driving aside! Why? What for?
I’ve attached two screens , where you can see better.

Why it cant be just lifted in Z direction, if we lift this like this?
Both are arcs. NOT straight line

SketchUp does not recognize your #3 as an arc. Even if an extension is used to attempt to convert it to an arc it does not become an arc. The edges of your #3 do not lie in a single plane.

The #3 is an arc. It also arises as a result of cutting a cylinder with a plane. In the attachement You can see that this is an arc and it has his plane. When You make the intersection you will get exacty such a arc, but sketchup doesnt create arc from intersection , just lines.
But the problem is why during lifting of B point upright in Z direction sketchup builds some spherical arc whose points are shifted in Xand Y direction so it looks as the picture? ??? Why is like that ?

Cutting a cylinder obliquely with a plane does not yield an arc. It is part of an ellipse. To be considered an arc in SketchUp it must be part of a circle.

Additionally, SketchUp is a surface modeled and as such, tries to keep edges planar so that faces can be formed. In your example, your number 2 curve is still planar. You can tell that because a face is formed when you connect the ends of the curve with a single line. Do the same to your number 3 curve and no face is created.

Ok, Dave, so look at this example.
What did I do?
I’ve lifted the point B to B1 than I drawn the chord between A and B (called x) , than I’ve created the arc with equale to this X bugle, (the same plane which sketchup creates) .
Now You get the really arc, which looks from above much better than this created when lifting.
I just dont understand why the arc which is created when we lifting has so big bugle??
Why creating the surface is a priority?

When you raise the end of the arc is remains an arc and by definition the edges are planar. Both of those are things that SketchUp tries to maintain. What you seem to want to do is convert an arc to some other curve.

Some of the confusion here might be from not clearly understanding the terms. An Arc has a radius, it is a section of a circle. When you start with a 90˚ arc and move one end of it, SketchUp tries to preserve the arc, so the result in your second example is because you have made a 90˚ arc larger in size, so it’s radius must also grow to stay an arc. See how when I inference the second example with the line tool the inference engine still shows me the center of the arc, and not with the third example? Your third example is part of an ellipse, it does not have a set radius and so cannot be an arc. Here, I take the second example which is still an arc and recreate a circle, this would not be possible with your third example.

The distortion you see is because the radial degrees of the original arc you start with is preserved.