At their Worldwide Developers Conference in June, Apple gave a preview of how all of their OSs will look this fall. They have a new style for some UI elements that they’re calling “Liquid Glass.”
Here’s the part of the keynote where they introduce it.
Here’s a 20-minute video that gives an overview of Liquid Glass.
Here’s a 12-minute video specifically about app icons.
And here’s a 15-minute video on how to make icons using their new app, Icon Composer.
You can download Icon Composer here. It’s free, 24 MB, and you don’t need Xcode or a developer account to use it. It runs on macOS Sequoia 15.5 and up.
Here’s where you can download the templates that are shown in the video. This also includes a script so you can easily export from Illustrator.
And finally, here's a short video that I made that quickly shows how to go from Illustrator to Icon Composer.