Why is a bubble from e.g. a bubble blower round (and e.g. not square)?

Answer

Once the bubble has formed, no air can enter or leave the bubble. So the volume is immutable. The optimal structure of the air bubble minimizes the energy of the soap film. This energy is typically proportional to the surface area of ​​the bubble. So it is a matter of finding the shape with minimal surface area for a given volume. This is a known mathematical problem and the answer is that a spherical surface has the optimal shape.