This article applies to Arc for macOS and Arc for Windows.
Some browsers allow you to save a website or web app to your macOS Dock or Windows Taskbar as a Progressive Web App (PWA).
Arc for Desktop does not have support for Progressive Web Apps (PWAs).
However, this is a common Member request and may be something we add to Arc in future.
In the meantime you can use Favorites for easy access to your top web apps.