In 2015, the government returned Ai’s passport, and ever since, the artist, 61, has been an unusually frequent flier. For his 2017 documentary, “Human Flow,” he visited 23 countries, including Afghanistan and Greece, to explore the international refugee crisis. And last year, from his current base in Berlin, he flew back and forth to Los Angeles, preparing a trio of exhibitions, including one that marked the debut of Jeffrey Deitch’s eponymous gallery, which opened in L.A. in September. That space was designed by Frank Gehry, perhaps the only architect whose fame rivals Ai’s. Long based in Los Angeles, Gehry has made a major mark on his hometown, starting with his 1978 remodel of his own house using chain-link and galvanized, corrugated steel. But his irreverent buildings, the most iconic of which are clad in aluminum titanium or stainless steel, have also defined other cities — like a branch of the Guggenheim Bilbao, with swooping walls that echo the sailboats in the harbor nearby.