> Is Asahi installed side by side on a mac?

Yes, the installer automatically (and reliably) resizes partitions for you. A minimum of about 70 GB for macOS is needed (anything lower is still possible but unsupported).

> You pick it at boot?

There's a default choice that will boot.

> And how “install and just use” it is?

Probably one of the smoothest Linux installs I've had in 10 years or so, since you just run the installer from macOS instead of flashing ISO files to an USB drive.