It’s noon, and Arkady Volozh is ready to christen the first proper headquarters of his new company, Nebius Group NV. Six days before Christmas, the mood is festive, with pizza and cans of Heineken carted out for the 200 or so employees gathered in the empty building, a short drive from Amsterdam’s main airport. “It’s not an office,” Volozh, 60, tells his staff. “It will be a home or a club.” The chief executive officer isn’t a natural pep talker. He seems more at ease milling about the room afte...