TIJUANA, Mexico — A nurse who fled Cuba as part of the Caribbean nation’s largest exodus in more than six decades needed a place to stay in Mexico as she waited to legally enter the U.S. using a government app. A woman who had lived her whole life in the same Tijuana neighborhood was desperate for medical help after a dog attack left her with wounds to her legs.

A mutual acquaintance brought the two women together. Nurse Karla Figueredo stayed with Martha Rosales for three days in October 2023, waiting for a border appointment booked through the CBP One app and treating Rosales’ dog bites. When Figueredo left for the U.S., she got Rosales’ permission to give her name to other migrants.

Word quickly spread and Rosales made her home part of a roster of at least three dozen migrant shelters in her hometown on the U.S.-Mexico border, temporarily housing people who use the CBP One app.

“I told God that if they didn’t amputate my feet, I would help every Cuban,” said Rosales, 45, who was using a wheelchair after being attacked by five dogs until Figueredo helped heal her wounds.

CBP One has brought nearly 1 million people to the U.S. on two-year permits with eligibility to work but could go away once President-elect Donald Trump takes office.

Figueredo, 25, now works as a medical assistant in the Houston area and keeps in touch with Rosales, who quit her job as a bank cleaner to focus on her migrant shelter. The people Rosales houses, mostly Cubans, refer to her as “’Tía Martha” (Aunt Martha) as she cooks pancake breakfasts, throws birthday parties and shuttles them to their CBP One appointments.

Supporters say CBP One has helped bring order to the border and reduced illegal crossings. But Trump has said he would end it as part of a broader immigration crackdown. Critics say it prioritizes a lottery system over people who have long lived in the U.S. illegally while paying taxes and people who have waited years for visas.

Dayron Garcia, a doctor in Cuba who heard about Rosales from a nephew, applied with his wife and children and plans to settle with a friend in Houston. He said Rosales’ house “feels like family” and that “CBP One has been a salvation.”

“It’s a guarantee,” Garcia, 40, said. “You enter with papers, with parole.”

U.S. Customs and Border Protection debuted CBP One near the end of Trump’s first term as a way for customs brokers to schedule inspections and for visitors with short-term visas to extend stays.

The Biden administration extended its use to migrants to replace an opaque patchwork of exemptions to a pandemic-related asylum ban that was then in place.

CBP One is popular with Cubans, Venezuelans, Haitians and Mexicans, likely because advocates in their communities promote it.

Illegal border crossings by Cubans plunged under CBP One from a peak of nearly 35,000 in April 2022 to just 97 in September.

Demand for appointments has far outstripped supply, with an average of about 280,000 people competing for 1,450 daily slots toward the end of last year, according to CBP. Winners must report to a border crossing in three weeks.

Migrant shelters along Mexico’s border with the U.S. are now occupied primarily by people seeking the online appointments.

Rosales’ house is in a neighborhood with ramshackle homes where old tires are stacked to stop flash floods. Migrants watch television, play billiards, do chores and look after their children at Rosales’ house or a rental home nearby. Those who don’t yet have appointments work their phones for slots made available daily at eight U.S. border crossings with Mexico, a task likened to trying to buy Taylor Swift concert tickets.

Rosales works throughout the night. A helper drives to the airport in an SUV Rosales bought with retirement pay from her bank job.

Shortly after midnight, she shuttles guests from her house to Tijuana’s main border crossing with San Diego for the day’s first appointments at 5 a.m. She chats with them, smiles for photos and hugs people goodbye.

By 3 a.m., she is at a television station for a four-hour shift cleaning the newsroom and fetching coffee for journalists, who give her the latest information on immigration and the city.

She checks her phone for migrants needing shelter who heard about her on social media or from friends and family. Her contact list identifies them by size of party and appointment date: “3 on the 16th,” “6 on the 17th.”

Rosales, one of 13 children, dropped out of school in third grade. Reading the Bible taught her enough to barely understand texts, which she generally responds to with voice messages or calls.

Enrique Lucero was Tijuana’s director of migrant affairs when she came to City Hall for advice. He helped Rosales establish a legal entity to raise money and made himself available for emergencies, such as when a woman missed her CBP One appointment to give birth. Lucero talked to CBP to make sure the woman and her baby got in.

“She worries about them. She cries for them,” Lucero said.

Border arrests of Cubans increased during the COVID-19 pandemic and after anti-government protests in 2021. Nicaragua had recently eased rules for Cubans to fly from Havana, allowing them to avoid walking through the Darien Gap, a dangerous jungle in Colombia and Panama. By the spring of 2022, Cubans eclipsed all nationalities but Mexicans in illegal crossings.

“CBP One came like a gift from God,” said Yoandis Delgado, who flew to Nicaragua in 2023, paid a smuggler $1,000 to reach southern Mexico and was repeatedly robbed by Mexican authorities while trying to reach the U.S. border. “CBP One gave us a sense of possibility, of hope.”

Delgado, a cook in Cuba, said Rosales’ home and neighborhood don’t stand out for people seeking to prey on migrants, giving a sense of security he wouldn’t get at hotels or other shelters.

“She lives in the same condition that we do, not any better,” Delgado said after a pancake breakfast. “She cries for everything that happens to us, for what we have suffered to get here from Cuba.”

Biden administration officials portray CBP One as a key success in its strategy to create legal pathways at the border while deterring illegal crossings. They note people in life-threatening circumstances can come to a border crossing without an appointment to plead their case.

Anxiety is spreading among migrants in Mexico who fear Trump will end CBP One. Even those in the U.S. are uneasy because parole expires after two years.

The Trump transition team didn’t respond to a question about CBP One’s future, but his allies say it’s overly generous and encourages immigration. A bill that stalled in the Senate in 2023 would have prohibited using the app to admit migrants.

Figueredo, the nurse who helped Rosales, plans to get a green card under a 1966 law that applies to Cubans. She says she and her partner, a barber, came to “continue to grow professionally and support our future children.”

She writes Rosales often, telling her that her job is “crazy” busy and asking about her health. “I hope you’re very happy,” she wrote.