Booting HA From an SSD on a PI4

The term hass.io has been deprecated, this method is now called Home Assistant OS. Here’s a good overview of the different installation methods.
Just for your information Home Assistant OS and Home Assistant Supervised also run in docker. All the add-ons are just docker containers modified to work nicely with the HA eco-system, stuff like ingress etc.
The only method that doesn’t use docker is the installation in a venv.