Macs have rounded corners on the desktop. I.e 4 corners. It looked awesome in the CRT days because of the shape of them. These days on flat lcd screens the geometry is so good that they aren’t required to provide the same function as in CRT days but some people still like the look. I do, but i dont run extra software if i can help it especially for an aesthetic change. I used a tool like this is the past but there were incompatibility’s.

Modern MacBooks models have rounded corners at the top right and top left corners of the screen, see [0]. The bottom corners don't do this and external displays don't do this. This is a stylistic choice to match the curvature of the laptop's edge. If you move your mouse along that corner it doesn't disappear behind the curve, it follows the curve.

[0] https://www.apple.com/v/macbook-air/x/images/overview/mac-pl...