I'm not sure that's true that it's (always) done in software. For example, the newest Framework Laptop 13 has higher resolution screens that come with rounded corners, because whatever supplier had a large stock of them lying around. If it was just software, the concept of having a "rounded corner screen" from a supplier wouldn't exist.

I'm pretty sure it's a screen cutout - the corners are still rounded when fullscreen zooming, and on asahi linux. There's also another cutout for the notch at the top of the screen. Also, it's not a great panel and the blacks aren't very good - you can still see light coming from the monitor with a full back image in fullscreen and it changes based on brightness.

So if it is software, then it's very low level and with darker blacks than anything else on screen, and has a super tiny bezel right at the corner. I'm pretty sure it's a cutout.