It was 1:53 a.m., and Peter Fink was on a barren mountain plateau near Campo, Calif., passing out blankets to people from four continents who had arrived there under the cover of night.

This was a nocturnal ritual for the 22-year-old, dressed in a ball cap and a wool overshirt, whose perch — just over 300 yards up a rocky incline from the United States-Mexico border wall — had become a round-the-clock boarding space for people who had crossed unlawfully onto American soil.

With Mexico’s armed National Guard now stationed at the most popular crossing sites along southeastern San Diego County, migrant routes have shifted further into the remote wilderness, where people face more extreme terrains and temperatures with little to no infrastructure to keep them alive.

For migrants who were aiming to be apprehended by U.S. Border Patrol agents and begin applying to stay in the country, Mr. Fink’s makeshift camp, a dirt patch under the lattices of a high-voltage tower, had become a first stop, where modest rations of donated food, water and firewood helped migrants survive while they waited for agents to traverse the landscape and detain them before their health languished dangerously.

At this site and others along the border, migrants have waited for hours or sometimes days to be taken into custody, and a Federal District Court judge ruled last week that the Border Patrol must move “expeditiously” to get children into safe and sanitary shelters. But unlike outdoor waiting areas that had arisen in more populated areas, Mr. Fink’s site had no aid tents or medical volunteers, no dumpsters or port-a-potties — just a hole that he had dug as a communal toilet, and Mr. Fink himself.

By the morning, there were Indians, Brazilians, Georgians, Uzbeks and Chinese.

Officials say federal funding and personnel are far too limited to keep up with the influx of border crossings in the region, and operations like these have become a source of great tension in San Diego County.

Asked whether he worried that his humanitarian aid might encourage more people to come unlawfully, Mr. Fink shook his head.

“People do not spend their life savings and risk the lives of their children so they can taste these peanut butter and jelly sandwiches,” he said.

Peter Fink is blond and fresh-faced, and grows a beard just to look his age. He grew up in the Pacific Northwest and learned Spanish working a summer job picking cherries. Captivated by the immigration crisis in 2020, he spent months in Arizona, walking over the border to volunteer at a Sonora migrant shelter by day and, by night, earning an international studies degree online, using free Wi-Fi at a local McDonalds.

He did not create this mountaintop camp; he found it. A local man had noticed fires burning on the plateau each night, and Mr. Fink, a wildland firefighter and avid camper who was traveling through the region, volunteered to spend the night on the plot in a tent to see what transpired. Within hours, over 200 migrants came on foot — among them pregnant women, children and elderly people — huddled together in the biting wind.

Word spread through the southern communities of what’s known as the Mountain Empire, an area so isolated that the small desert town of Jacumba Hot Springs (population 857) 30 miles away, became operation headquarters. Volunteers gathered firewood from the discards of an ax throwing venue and a live-edge table maker. An abandoned youth center was used to sort nonperishable donations. A shipping container in someone’s yard became a sort of depot for crates of water and tarps.

After that first night in early March, Mr. Fink spent another, then another. He pitched a series of four-person tents in a tidy line, cramming 10 people into each when the wind became particularly unbearable. He used white paint to label the drawers of old office filing cabinets in four languages, denoting rations of applesauce for children and formula for infants. He established guidelines for his campsite: one snack per person; no littering; conserve firewood; women and children receive priority in the tents.

On this day, the sun was almost directly overhead when Mr. Fink peered out through his binoculars and saw a couple being dropped off by an unmarked vehicle on a dirt road in Mexico and trekking through the arid brush toward the United States. The woman began slowing down. She was visibly pregnant.

Mr. Fink grabbed two water bottles and began his descent into the canyon below, waiting for the two a safe distance back from the border wall so as not to encourage them. Once on U.S. soil, the woman panted heavily and lowered herself to the ground. Her husband squatted in front of her and took her face in his hands.

“Está bien?” he whispered, wiping the sweat from her brow. She nodded.

For a moment, there was silence. Then Mr. Fink asked in Spanish where they were from (San Salvador), how soon the baby was due (one month) and whether the two had been extorted for cash by Mexican authorities on their way to the border wall. The couple said they had not.

“Buena suerte,” he said.

He led them on the ascent to camp, passing abandoned bags and clothing, and using footholds he had carved into the earth with a technique he had learned fighting wildfires. As soon as they arrived at the camp, he turned and began sprinting down into the valley again. He had spotted a young girl in polka-dot pants and a ponytail wandering with her mother, and could see that they were about to make a wrong turn.

Once the girl, Briana Lopez, 5, arrived at the camp, she ate Welch’s fruit snacks from Mr. Fink, and spoke by phone to her father, still back home in Guatemala.

“How are you, my child? You happy?” he asked in Spanish.

“Bien!” she said. “Sí!” Good! Yes!

Her parents discussed how she and her mother might navigate immigration detention once they were apprehended. Briana chimed in, excited — she believed they were going to Disneyland.

The last group of migrants was picked up by dusk, and Mr. Fink crouched in his tent, munching on a piece of pita bread and arranging donation drop-offs via his cellphone.

This was around the time he usually went to sleep, hoping for a few hours before the first overnight wave arrived. But in the distance he heard exasperated breaths, and a woman appeared alone, collapsing into his arms, weeping.

Her travel companions had left her behind, she said, following an underground railroad track and bearing too far to the west, disappearing into the wilderness. Now they were missing.

Mr. Fink climbed to the highest point on the rocky ledge, cupped his hands around his mouth, and shouted in Spanish: “Here, we have water and food! Do not be afraid — come this way!” his voice echoing through the valley. “Hey, welcome to the United States!”

He wrapped the woman in a blanket as she waited. “Dios te bendiga,” she said. God bless you.

Finally, her two lost companions climbed over the crest from the other side of the plateau, sobbing and wrapping their arms around her. Mr. Fink packed a bag for each of them as they followed Border Patrol orders to strip down to one layer of clothing and climb into a government van.

At 8:13 p.m., the site was silent again, except for power lines buzzing overhead and dogs cooing their evening songs on the Mexico side. In the darkness, Mr. Fink sanitized and tidied the tents, then lit garden lights and glow sticks along the path up to camp for those who would arrive in the night.

Within a week, Mr. Fink would depart for the Northwest, where planting season for sorghum and amaranth would begin, and where he had landscaping and construction jobs waiting for him. But his tarps, firewood and filing cabinets atop the mountain remain, and supplies are restocked periodically by volunteers.

When a group of Colombians were released from Border Patrol custody into the United States the following week, an aid worker heard them discussing “an angel” who had kept them alive and won their hearts — “un güerito” who spoke very good Spanish, they said, and who they had found hanging out in a tent.