How is the blurring of the preview view realized?

How is the blurring of the preview view realized? Is it through the image rendering or directly rendering the Bezier curve?Is there an interface to realize it?

That is a CoreImage filter on the view/layer.
here is some code that might help:

CGFloat radius = 30;
_blurFilter = [CIFilter filterWithName:@"CIGaussianBlur"];
[_blurFilter setDefaults];
[_blurFilter setValue:@(radius) forKey:kCIInputRadiusKey];
if (@available(macOS 10.14, *)) {
	view.layer.filters = @[_blurFilter];
}
else {
	view.contentFilters = @[_blurFilter];
}