It was absolutely not restored. The only things restored were things deleted specifically by the blip. Anything Thanos destroyed earlier - like Vision, Gamora, and Xander - are very much still gone.The Nova Corps are an intergalactic police force and Xandar is an entire planet and the capital of the Nova Empire. The ships used in the battle of Xandar were not every last ship in existence.
Xandar was destroyed by Thanos when he stole the power stone from them, but it was restored when the Avengers undid the blip.