The I.R.S. says that Bristol Myers Squibb, America’s second-largest drug company, has engaged a tax-shelter setup that has deprived the United States of $1.4 billion in tax revenue. The Biden administration is looking to put an end to such practices to pay for its policy ambitions, including infrastructure like improving roads and bridges and revitalizing cities. We look at the structure of these tax arrangements and explore how, and whether, it’s possible to clamp down on them.