It is Mexico proper, of course, but Tijuana and San Diego seem to exist in a nebulous nation all their own, one not so easily defined by walls. Tijuana feels like it’s on the outskirts of San Diego because, well, it is; some parts of San Diego, like the Mexican-American enclaves of Chula Vista or Barrio Logan, could be an extension of Tijuana.

“They call Chula Vista the nicest neighborhood in TJ,” said Gibran Huerta, a creative director in San Diego whose company has employees in both cities. I’d run into him at One Bunk, which he’d helped design. “When you grow up in San Diego, you grew up with TJ in your life,” he said. “When you live here you don’t really see the border. It’s a crazy cross-border lifestyle for those who aren’t afraid to cross.”

On Sunday, Mr. Huerta invited us to crash his group’s breakfast plans. “Go where the people are,” he said, and according to him, that morning the people would be at an unassuming, unnamed cart at the intersection of Avenida C. Niños Heroes and Calle 4ta, which ladles vats of steaming hot birria, a beef stew from Mexico’s Jalisco province. We weren’t even sure if the stand would be there when we arrived, but there it was, doing brisk business at 10 a.m. on a Sunday. Mr. Huerta jostled through the crowd and ordered a round of tacos: heaps of birria encased in crunchy, flaky tortillas unlike any I’ve ever eaten before.

“Tijuana is not a picturesque place,” said Mr. Huerta. “There’s a lot to eat, not to see.”

Except there is one thing. While the specter of the wall looms large in politics, in Tijuana, it’s been a fact of life since the 1990s. The border wall at Playas de Tijuana is not rhetoric but very real, an imposing slatted barrier 18 feet high that extends into the sea, separating Tijuana from San Diego County’s Imperial Beach. On the Mexican side, it’s a lively scene, with people gathering to walk on the beach, eat fresh coconuts, and take selfies with a wall covered in colorful murals. “Vive Libre Sin Muros.” “Love Trumps Hate.” “Earth Was Not Meant to Have Walls.” And on each side of the wall, families who are separated by deportations and tenuous visa status gather for teary reunions through the barrier.