By Zhaoyin Feng & Joshua CheethamBBC News

For the global Chinese community, WeChat is more than a chat app: it is often the primary means of staying in touch with friends and family back home.

So the decision last week by US President Donald Trump to order American firms to stop doing business with WeChat has sent shockwaves rippling out.

"WeChat has become the 'it' tool for Chinese-speaking people, no matter where you are around the world," one Shanghai resident told the BBC.

The billion-user app is primarily a social networking platform but can be used for a host of daily activities like shopping, gaming and even dating.

But WeChat has a less innocent side, and is seen a key instrument of China's internal surveillance apparatus.

A Wuhan doctor, Li Wenliang, was reprimanded by police last December after he sent messages to colleagues on the app, warning them about China's initial coronavirus outbreak. Toronto-based research group Citizen Lab also found WeChat had been censoring information about Covid-19.

WeChat's success is partly owed, as well, to a ban on American-made apps like WhatsApp and Instagram in China.

In an executive order, President Trump labelled WeChat a threat to US national security and accused it of gathering "vast swathes" of user data, threatening Americans' personal and proprietary information.

WeChat's owner, TenCent, has been ordered to sell the app by mid-September or face a ban on US operations.

The move to block WeChat, a prominent example of China's tech innovation, is seen by many Chinese as an attack on their culture, its people and state. In response to President Trump, China's foreign ministry has accused America of using national security as a cover to exert hegemony.

The Chinese diaspora in the US has been shocked by the move, and many people are worried - not just about keeping in touch with loved ones, but what this means for China-US relations.

'An unwelcome signal'

Jennie, 21, is a student at the University of California, and learned about the order while browsing WeChat.

"At first I didn't believe that it is true," she told the BBC. "Then I just felt very angry."

Jennie spends around four hours a day on WeChat, using it to contact people in the US and China. It is also a vital source of information and she spends a lot of time reading articles published on the public accounts of Chinese media, content creators and businesses.

On the anniversary day of Tiananmen Massacre, Jennie made a one-sentence post of commemoration. It was quickly removed, and her entire public account vanished.

She told the BBC she is "very worried" that WeChat will share her information with the Chinese government, but strongly opposes America blocking the app.

"It'd be similar to what China does - to censor," said Jennie.

She used to publish on her own public account, until it was censored by WeChat two years ago.

Jennie believes that there should be alternatives to manage threats posed by WeChat, rather than banning it outright.

"I wanted to study in the US because of its openness, but this move has burst my bubble."

This sense of disappointment is shared by other Chinese immigrants in the US.

"I used to think America is culturally inclusive," says Miley Song, a Chinese immigrant in California. Washington's move sends "an unwelcome signal" to Chinese immigrants in the country, she believes.

The 30-year-old stay-at-home mum often uses the app to connect with her parents in China, who were in a panic after hearing about Mr Trump's executive order.

But Ms Song says she is cautiously optimistic. "The ban seems very vague, I think it may be difficult to ban WeChat entirely," she says, "We'll wait and see."

While not particularly worried about the ban, she is concerned about what it means for her future in the America.

In the midst of a pandemic, and with presidential elections on the way, Ms Song thinks the Trump administration is trying to divert attention away from rising death tolls and falling polling numbers.

"Otherwise, why has Trump focused on cracking down Chinese apps now?"

'It's fully embedded into people's lives'

There is also concern among those who have returned to China after living and studying in America.

Rachel spent 10 years in the US, many of them as a student in the capital, Washington, DC.

Now home in Shanghai, WeChat has become "fully embedded into people's daily lives," she told the BBC.

"If you live in China, you cannot go anywhere without two major apps: one is WeChat, the other is AliPay," said Rachel. "If you want to buy a bottle of milk, you open your WeChat Pay or Alipay to scan the QR code and pay, and most stores don't accept cash."

WeChat is also being used as a tracking tool to help the government with containing the spread of coronavirus.

"If, say, I'm going to a department store, at the door usually the guard will ask you to show your QR code," said Rachel. "If it's a green code, which shows you haven't been to any high-risk place in the last 14 days, yes you can come in, but if it's a yellow or red code, you cannot."

While President Trump's order will have little impact on her daily activities in China, Rachel said it may become harder to connect with people in the US. As a result, she said some are exploring alternatives like communication app Line, or VPNs - virtual private networks which mask your computer's location.

"It's sad that it's going down this way," said Rachel. "I see both sides, there is always good and bad in both societies, and I want to be neutral but it's harder and harder to become neutral."

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