Here and in mining (which the authors also put in the secondary sector), you see a kind of U-shape: First, as countries emerge from deep poverty, the manufacturing share of employment increases.

Many illegal immigrants naturally find employment in lower-paying, or “secondary sector” jobs. Initially, this can stimulate growth in primary sector jobs for native workers.

As countries become richer, employment in the primary sector, agriculture, declines. That’s because the industrial revolution kicks off, and employment in the secondary sector, industry, increases.