Because the ARCVM, the Crostini and the ChromeOS is tightly integrated, you don't actually feel like using 3 OS. under the hood, they work quite seamless. You can right click install a .Deb file, and it will be registered into a the main menu, alongside your ChromeOS PWA, and android apps.
It does not feel emulated either. Copy paste is smooth, UI looks blended. dark mode on the main ChromeOS makes debian and android dark mode too.