TIJUANA, Mexico — A neighbor of Esther Arias is building a new fence — a very high one — and the construction is taking a bit of a toll.
“Sorry about the mess,” she said, waving at the scrap metal strewn over her back patio, the remnants of the neighbor’s old fence that was taken down to make room for the new one.
Ms. Arias did not seem too bothered by the disruption, but even if she had been inclined to fight, a battle would have been hopeless.
She lives right on the border in Tijuana, Mexico, and her neighbor is known to have a lot of power: It’s the United States government.
She said United States Customs and Border Protection officials promised she would not be disturbed while the work was being done.
“I respect them and they respect me,” she said.
Tijuana is wedged hard against the straight line drawn in 1848 to divide Mexico and California. The city’s urban sprawl now extends some 15 miles east from the Pacific Ocean to the spot where the last houses seem to scatter into the scrub land.
There, the border wall stops short, like a book snapped shut, as the dividing line begins to rise into rugged mountains.
Of all the houses along the border, the cement house where Ms. Arias, 52, raised five children may be pressed up closest of all to the barricade that now defines the border. The United States fence does double-duty as her patio fence.
But the divide wasn’t always so stark.
For a long time, the barrier was more of an afterthought, at its most formidable points just some barbed-wire strung between posts. Then in the early 1990s, the United States used Vietnam War-era steel helicopter landing mats to build a wall.
Over the years, that first wall, now splashed with murals, has metastasized.
A second fence stretches behind most of it, and between the two lies a no-man’s land of cameras, sensors and floodlights.
This year the border agency began to replace the old metal wall. The new sections, between 18 and 30 feet high, are built of closely spaced steel posts topped with a steel plate designed to deter climbers.
Despite these changes, ask almost anyone in Tijuana about the wall, or “la linea,” and you are likely to be met with a shrug: The wall is always present, but not a preoccupation.
“We live very comfortably here,” said Elizabeth Quintana, 73, who runs a small restaurant from her house on a dead-end street that runs into the wall.
When she moved to Tijuana’s Libertad neighborhood in 1972, the border was just a few shin-high cement markers. Her only complaint about the giant steel bars that mark the line these days: To install them, she said, “they pulled up all the trees.”
Daily life in Tijuana is defined less by the wall as an impenetrable obstacle than by the ebb and flow of movement across it — or, for many, the distant hope for such a journey.
As many as 150,000 people travel north toward San Diego on foot or in cars every day through two border crossings. Thousands of trailers roll through a separate crossing, carrying Mexican-made goods on their way to American stores and factories.
This passage is a daily ritual for many who are United States citizens, or Mexican citizens with green cards or visas that allow them to move freely.
The first commuters arrive hours before dawn, their cars rolling forward along two dozen parallel lanes at the San Ysidro crossing as drivers check email, apply makeup, knit or extend a hand to caress a child snoozing under blankets, bound for school in the United States.
“How do I feel?” said Asheila Ramírez, 40, who shuttles several times a week between her aunt’s house in Chula Vista, Calif., where she works cleaning houses and driving for Uber, and her mother’s house in Tijuana.
“It’s normal for me,” she said. “Really, I’m used to it.”
On their way, these commuters pass vendors selling fruit drinks, churros and burritos, amputee beggars on crutches and merchants hawking religious trinkets.
“I live here,” said José Felix, 49, who drives a cab in California. “I pay my taxes over there.”
The dream of finding a way into the United States to escape poverty, violence and persecution has for decades drawn people to Tijuana from all over Mexico and Central America, and as far away as West Africa.
Some hope to be granted asylum or a different legal route; a good many will pay smugglers to take them across.
While these migrants are always a presence in the city, every so often, they burst into public attention, as they have in past weeks with the arrival of more than 6,000 Central Americans traveling in a caravan from Honduras.
Their long journey was halted by the wall, and their increasingly desperate situation underscored just how jarring the disconnect is between those who can cross and those who are blocked.
One recent morning, as these migrants lined up to put their names on a waiting list for an asylum interview, a group of day-trippers who arrived on foot from San Diego to begin a Tijuana tour snapped photos of the weary queue.
Then there are moments of genuine menace along the border.
The arrival of the migrant caravan spurred border agents to full readiness.
Dressed in riot gear, they have held several training exercises at the San Ysidro and the Otay Mesa ports of entry in recent days.
On a Sunday in November, several hundred migrants tried to cross the border en masse. The Mexican federal police barricaded the crossing and pushed them back as United States border agents fired tear gas from behind the wall.
While that push for the border played out before the international news media, crossing the border illegally is largely a matter of stealth.
Late one recent afternoon in Tijuana, smugglers could be spotted using four homemade ladders to ferry a couple of migrants up and down both walls in the deceptive light of dusk.
As a border patrol car approached, one of the smugglers, in the no man’s land, darted up a ladder, swung himself over the wall and climbed down back into Mexico.
An officer removed the ladders the smuggler had left behind, but everyone knew they would soon be replaced.
Such elements of the absurd are inescapable here.
A few years ago a Japanese art collective built a treehouse above Ms. Arias’s back patio with a view over the border wall.
The artists gave a cheeky name to their project, where Ms. Arias’s grandchildren now play, looking out over a country they never visit: U.S.A. Visitor Center.