If you do what Donald Trump has never done, and walk across the Paso del Norte international bridge that connects El Paso in Texas with Ciudad Juárez in Mexico, it is like entering a hall of mirrors. Everything is the same, yet not the same.

On either side of the bridge there are the same broad streets lined with shops, the same people milling around viewing wares. But the shop signs are different. On the American side of the bridge they are in Spanish; on the Mexican side, in English.

In El Paso they advertise American consumer brands – Nike, Adidas, Hermes, Prada. In Juárez they hawk cheap pharmaceuticals, dentistry and wrinkle removal.

That’s what trade in the modern world looks like. Thousands of Mexicans pouring north across the bridge in search of a commercialised taste of the American dream, thousands of Americans crossing south seeking eternal youth in the form of cheap drugs, porcelain teeth and botox.

That’s 23,000 pedestrians every day, more than 7 million a year. United by a common language, no matter whether in English or Spanish: the language of fear.

Fear that Trump will go through with his threat, issued on Friday through one of his tweets and reiterated since then, to close the border. Fear that with one flick of his executive pen, Trump could sever ties between the two cities that stretch back almost 400 years, shattering hundreds of thousands of lives in the process.

People walk across the Paso del Norte international bridge into Ciudad Juárez on the US-Mexico border. Photograph: Guillermo Arias/AFP/Getty Images

It doesn’t matter which side of the bridge you stand. In this hall of mirrors, everyone says the same thing.

“This would be the end for me,” said Carmen, who has a clothing store on the El Paso side. “One hundred percent of my customers are from Juárez.”

A few shops down, Marcela Coallo is selling handbags in a shop called Bolsa Coketa. “We’re all scared, because we all depend on people from Mexico,” she said. “President Trump talks about wanting to help American families, so why is he threatening our livelihoods?”

How long could Bolsa Coketa survive if the border closed?

“One week perhaps,” she said. “That’s how serious this is.”

Faced with the same question, Juan Aguilera, a pharmacist on the Mexican side of the bridge, has an equally grim answer. About 80% of his customers, he said, are US citizens, come to buy pharmaceuticals at knockdown prices.

How long could he survive with a shuttered border – a week, a month, a year?

He laughed.

“Days,” he replied. “Just a few days.”

Over the weekend, Trump said that if he does go ahead and close the border, “we’ll keep it closed for a very long time.”

A view of the Santa Fe Bridge (Bridge Paso del Norte International) connecting El Paso and Ciudad Juárez on June 20, 2018 in El Paso, Texas. Photograph: Brendan Smialowski/AFP/Getty Images

If the language of fear is not loud enough to reach Trump in the Oval Office, then perhaps the language of numbers carries better. Some $82bn in trade crosses between El Paso and Juárez every year in an intertwining of economies that makes this the fourth largest manufacturing hub in North America.

Writ nationally, that amounts to $557bn in annual trade between the US and Mexico. And it supports 5m American jobs, many of them in the manufacturing sector, and many located in precisely those states – such as Pennsylvania, Michigan and Wisconsin – that handed Trump his presidential victory in 2016.

For a president who was put in the White House in no small part because of his pledge to increase American trade and protect American manufacturing jobs, that’s an impressive figure.

“This makes no sense as a manufacturing plan,” said Tom Fullerton, professor of economics at the University of Texas at El Paso. “Talk of closing the border is putting US manufacturing at risk by potentially disrupting the supply chains that support these industries.”

Commuters show their visas to border patrol agents to cross into El Paso from Ciudad Juárez. Photograph: José Luis González/Reuters

Fullerton warned that in the short term it could lead to bankruptcies, and in the long term to firms pulling out of the border region. That posed an existential threat to the “maquiladora” industry: the complex interrelationship that generates hundreds of thousands of jobs on both sides of the border through American-owned factories located inside Mexico that assemble auto and other parts initially built in the US.

“Trade is not a zero-sum gain, where a job in Mexico is taken away from the US,” said Jon Barela, CEO of the Borderplex Alliance that represents 250 cross-border businesses and manufacturers. “Quite the opposite: for every four jobs created in Juárez there is one high-wage job created in El Paso.”

Once you leave the hall of mirrors and cross back over the Paso del Norte into El Paso, you can see what Trump’s drastic rhetoric is all about. If you look down on the side of the bridge you can see behind a wire fence that a group of young women and infants are being held, sitting on the ground.

These are the Central American asylum seekers who Trump says are “invading” America and that must, at all costs, be stopped. From 50ft up on the bridge, looking down on them, they seem too small and helpless to merit so much agony.

“He says he’s doing this to protect the US, but it doesn’t seem logical,” said Carmen in her El Paso clothing store. “Why does he have to be so dramatic? This looks like a provocation to me.”