Gonna need you to elaborate a bit here

If you just mean simply how they created the design: some designers will have put proposals together and built the icon designs up in something like illustrator

If you mean how Apple can change the icon based on settings, if it’s anything like the Android adaptive icon feature we’ve had for a few years, it’ll be a main colour transparent PNG of the main part of the icon and a 2nd monochrome version of the same. Then the user & OS can do what they like when rendering the icon, regarding icon borders and background colour

I think the answer is in this article: https://arstechnica.com/gadgets/2025/09/macos-26-tahoe-the-ars-technica-review

Poorly

Drugs?

Icon Composer by apple makes it piss easy