DETROIT—General Motors’ profit fell 4.5 percent in 2020, but a strong second half more than offset the effects of pandemic-related factory closures and a costly airbag recall. The Detroit automaker said Wednesday it made $6.43 billion as demand for its vehicles surged late in a year dominated by coronavirus upheaval. In the fourth quarter, the company made $2.85 billion. After GM’s North American factories reopened in May, the company ran many of them flat-out but couldn’t make up all the lost production. As customers returned to buying again, inventory was short and GM’s U.S. sales for the year fell 12 percent from 2019. But because buyers bought more expensive trucks and SUVs and loaded them with options, the company was able to turn a big profit. Excluding one-time items, GM earned $4.90 per share in 2021, beating Wall Street estimates of $4.40. Revenue for the year was $122.49 billion, which …