What is and how it appears this kind of circle in this tutorial?

Question

Im very very new on this, i tried doing this tutorial of a food shop but in minute 2:12 https://youtu.be/U2LHxud-CVg?t=132 they press shift +d that creates a copy but from nothing appears some kind of circle or oval

then in separate loosen parts it can be seen after using shift + D and P but i cant really understand how it was there, theres no voice in video explaining so i dont know whats this or what key they used, this doesnt appear to me after selecting + shiftD + P

it looks like a rectangle to me, not like a oval after doing exactly what i could see, someone could help me pls im stuck here

Answer 1

The square becomes an oval because of the Subdivision Surface modifier:

The Subdivision Surface will create virtual topology in order to round the shape of the object. With a square it will create this circle. Here is what we can see if I display the wireframe and deactivate the Optimal Display option of the modifier, we can see the topology (it's only a virtual topology as long as you don't apply the modifier):

If I duplicate all the top faces instead here is what it gives. This time it doesn't look like a circle because you have these 4 support edges along the edges that will prevent the central square to be circular, only the corners will be round: