In the US, the national average of starting salary of teachers is $38,617, while the average salary of experienced teachers is $58,950, according to data from the National Education Association.

Even on 51Talk, where the majority of tutors are from the Philippines, teachers pull in a decent sum of around 430,000 peso ($7,945) per year, which is much higher than the country's average annual income of 176,000 peso.

Chinese companies follow the local laws applicable to the tutors, who work in the capacity of independent contractors, hence pay taxes on their income locally. Typically, payments after tax deductions are made directly into their bank accounts in local currency.

"Besides earnings, flexibility is another important reason that foreign teachers apply for the part-time jobs as they can take advantage of their spare time to teach," said Lyu Senlin, founder and chief researcher at the Learneasy Times Online Education Research Institute, an industry research consultancy.

Online platforms' foreign teachers can arrange classes to suit to their convenience. They can log in and teach from any quiet place.

Prestigiacomo, who had a baby and couldn't work during the day, is now able to take care of her baby and at the same time teach six days a week. Catherina agreed that working like this was "perfect" because she could teach at a time that suited her.

"It's also a shining example of the economic globalization," she said. "Online education, as the latest form of internet economy, is overcoming the limits of geography and time zones."

For long, China has identified education as a top priority. Since 2016, the government has been investing over 3 trillion yuan a year in education. This accounts for around 4 percent of gross domestic product. Chinese parents have high expectations of their kids and are willing to spend big money for high-quality education.

Beijing's Jing Zhiqiang, 42, a father of a 9-year-old son, parted with 10,980 yuan ($1,500) for a set of 72 classes for his junior. The lad attends four classes a week on an online education platform.

The family spends 2,400 yuan per month for an online English course. That's half of Beijing's per-capita monthly disposable income of around 4,800 yuan last year.

For Jing, the first reason behind choosing the online course is that his children can take one-on-one personalized tutoring from native English speakers. This, he believes, will help the kid.

"Also, home-based tutoring is a great relief for both my wife and me as we don't have much time to pick up and send the child to tutoring institutes," said Jing. "Particularly in Beijing where the traffic is often terrible, we actually save a lot of road time... Time is money, isn't it?"

Zhou from PwC said, "It's apparent the biggest demand for foreign language tutoring comes from China. The market will continue to grow driven by demand for studying abroad, business trips as well as travelling."

His view is in line with a report from UBS Securities that the market scale is expected to exceed 714 billion yuan by 2025.

"Such a burgeoning business determines that the country will surely provide more and more flexible opportunities to foreign nationals outside China in the future, bringing more and more benefits for both Chinese and foreign economies," said Lyu.

In a sense, China-based online education startups could be said to promote international economic and cultural ties, given that employment and jobless rate are big concerns in many countries, and trade tariff disputes tend to sour people-to-people sentiments.

Kim Saylor, 52, an English-language teacher based in Texas in the US, has taught 450 students over 2,300 classes on VIPKid. She said she was pleasantly surprised to find that she had forged many warm relationships with Chinese families.

One of her students made her a birthday card and held it up in front of the computer camera to greet her. For her part, Saylor makes cupcakes and dispatches them in a special parcel all the way to China for her students' birthdays.

"I was so moved when he took out a cardboard violin, put in a CD and serenaded me," recalled Saylor.

"Opportunities to connect with students outside of the classroom make these connections so much stronger."