Seems Galileo has four different services that it offers. Open Service is the public service. If you understand the protocol and radio signals, you can build your own device to read it yourself.

High Accuracy Service used to be the commercial service, but is now just called HAS and is offered free of charge apparently.

Then there is Public Regulated Service for government bodies, and a Search and Rescue Service.

Part of it is the difference in the receiver hardware. Survey-grade GPS uses the phase of the signals as well as their timestamps to measure sub-wavelength resolution. This requires larger antennas and much heavier-duty processing hardware, and generally only works when stationary as well, so it's not really feasible for a mobile device.

There are two main improvements possible with GPS signals:

WAAS gets you to a pretty solid 1 meter accuracy.

RTK gets you 1 to 2 cm accuracy.