TL;DR: it's integrated with the rest of Cocoa. macOS cursors are provided by an NSCursor class which can either be one of the standard cursor image or provided

TL;DR: it's integrated with the rest of Cocoa. macOS cursors are provided by an NSCursor class which can either be one of the standard cursor image or provided through an NSImage object, which supports a number of data types (among the usual pixel-based formats like PNG or JPEG also PDF), it looks like applications can also register their own data formats via a class called NSImageRep.