How does an astronaut float in a space shuttle?

A Shuttle in orbit was not beyond gravity; it was inside it, and falling. A few hundred kilometres up, Earth's pull is only modestly weaker than at sea level, and it was exactly what bent the orbiter's path into a closed loop instead of a straight line. The vehicle moved sideways fast enough that the surface curved away beneath it as fast as it dropped. Crew, seats, checklists and escaped droplets of water all fell along the same curve at the same rate, so no relative motion appeared between them.

Weight is a sensation of support, not of gravity. On the ground the structure under your feet pushes up and the body reads that push as weight; in free fall the structure falls with you, so it pushes with nothing. Shuttle crews therefore worked in foot restraints and moved along handrails, because a firm shove against a wall would send someone across the middeck. Drinks were sucked from sealed bags, tools were tethered before use, and sleeping bags were strapped to whichever surface was convenient, since up and down were only a convention set by the labelling.

It was never perfectly still. Thruster and orbital manoeuvring burns, atmospheric drag on a large airframe and the crew's own movements produced small accelerations, which is why the environment is properly called microgravity rather than zero gravity. Full weight returned the moment something held the vehicle up: an engine burn, or the atmosphere itself during re-entry, when the orbiter became a glider and the crew felt their own bodies again. The Shuttle flew 135 missions between 1981 and 2011 under those rules.