The installer prompts you for how much you want to shrink your macOS partition and then installs Asahi to a newly created set of partitions.

You still need macOS to do any sort of firmware upgrade, so there is no blessed install process that completely replaces macOS.

Always alongside.

IIRC from one of the early posts that Marcan posted a couple of years ago, it's essentially impossible to completely get rid of MacOS due to the way the Apple Silicon boot process works.

afaik there has always been the option to install alongside during the installer. It might not even be possible to do otherwise if you're installing from macOS itself.

If you're talking about the Reg article, I did make it clear, and it only supports dual-boot for now.