HONG KONG—Asia’s sophisticated electronics supply chain and massive labor pool are two obstacles standing in the way of President-elect Donald Trump’s pledge to make U.S. companies bring manufacturing jobs home.

When Jabil Circuit Inc., the world’s third-largest contract manufacturer by revenue, needed to quickly ramp up production of its electronics components a few years ago, the company was able to add 35,000 workers in China in less than six weeks.

“In no other country can you scale up so quickly,” said John Dulchinos, vice president of digital manufacturing at Jabil, a St. Petersburg, Fla., supplier to companies such as Apple Inc. and Electrolux SA. “You have the ability to move quickly and there’s a really strong electronics supply chain in Asia centered around China.”

Jabil’s experience underscores the integral roles of China’s armies of migrant workers and Asia’s decades-old supply chain in global electronics production. It is an issue Mr. Trump will need to address if he wants to bring large-scale production back to a U.S. economy that Washington, D.C. think tank Economic Policy Institute estimates has lost more than 5.4 million manufacturing jobs and 82,000 factories between 1997 and 2013.

“I’m going to get Apple to start making their computers and their iPhones on our land, not in China,” Mr. Trump said in March, a theme he repeated throughout his campaign. “How does it help us when they make it in China?”