TikTok is restoring its service in the US after Donald Trump said he would issue an executive order when president to allow the app to continue operating.

It had shut itself down late on Saturday in advance of a Sunday deadline to divest its Chinese shareholders or face a ban, but resumed operations on Sunday, the day before Trump’s inauguration, saying it had received the appropriate assurances from the president-elect.

Why did TikTok shut down and reopen?

TikTok has been under threat from a US federal law requiring its American arm to shed its Chinese ownership – the app is controlled by the Beijing-based company ByteDance – by 19 January over fears the Chinese government could access the personal data of users and manipulate its powerful algorithm, determining what users see. The legislation forbade companies such as Apple, Google or Oracle from distributing or maintaining the app if ByteDance had not completed a deal by 19 January, amounting to a de facto ban.

TikTok had said last week it would “go dark” if the outgoing Biden administration failed to provide assurances. A vague statement from the Biden White House stating that it was down to Trump to enforce the ban was deemed insufficient and so TikTok pulled the plug.

Trump then stated in a post on his Truth Social platform that he would issue an executive order to extend the period of time before the law’s provisions took effect, with no liability for any company that “helped keep TikTok from going dark before my order”.

TikTok decided it had received enough clarity from the incoming administration, saying on Sunday Trump had provided assurance “to our service providers that they will face no penalties providing TikTok to over 170 million Americans” and that it would restore service.

TikTok’s chief executive, Shou Zi Chew, has denied Chinese state involvement with the app, saying ByteDance is not “an agent of China or any other country”.

Will Trump’s executive order work legally?

The law allows for an extension if there is a sign of progress in transaction talks to sell TikTok’s US operation to a non-Chinese owner, but it is not clear if that can happen after 19 January.

The supreme court has upheld the law, which was passed by Congress with support from Republicans and Democrats and was approved by Biden.

Republican lawmakers have said companies affected by the legislation must adhere to it, warning that anyone who violates it faces “ruinous bankruptcy”.

“Now that the law has taken effect, there’s no legal basis for any kind of ‘extension’ of its effective date,” the senators Pete Ricketts and Tom Cotton said in a joint statement. “For TikTok to come back online in the future, ByteDance must agree to a sale that satisfies the law’s qualified-divestiture requirements by severing all ties between TikTok and Communist China.”

Apple, Google and Oracle have been contacted for comment.

Will Trump still seek a change in TikTok’s ownership?

Trump said in his Truth Social post he would like the US to have a “50% ownership position” in TikTok “in a joint venture”.

He indicated that ByteDance could remain involved and that the venture could be between “the current owners and/or new owners” with the US getting “50% ownership in a joint venture set up between the US and whichever purchase [sic] we so choose”.

It was not clear what would constitute 50% US ownership. According to TikTok, Beijing-based ByteDance is already 60%-owned by non-Chinese investors including the US-based BlackRock and General Atlantic.

Why has Trump changed his mind on TikTok?

Trump had attempted to ban TikTok on security grounds by executive order in his first term as president but was unable to do so after it brought a successful lawsuit.

Last month he said he had a “warm spot in my heart for TikTok”, where he has 14.9 million followers.

The billionaire Trump ally and Republican donor Jeff Yass is also a major investor in ByteDance. His financial trading company Susquehanna International Group owns a reported 15% stake and Yass, a free speech-supporting libertarian, has described a TikTok ban as “anathema to everything I believe”.

Trump has said Yass “never mentioned” TikTok at a meeting between the men last year.

The incoming US president has also signalled his dislike of Mark Zuckerberg’s Meta, the owner of Facebook and Instagram, in relation to TikTok. “If you get rid of TikTok, Facebook and Zuckerschmuck will double their business,” he posted on Truth Social last year.

Meta has attempted to close the gap with Trump in recent weeks by relaxing moderation guidelines and installing a former George W Bush White House official, Joel Kaplan, in a senior role.

What does China say?

In 2023 the Chinese government said it would “firmly oppose” any forced sale of TikTok, and that a deal would require its approval.

On Monday China’s foreign ministry signalled a more open stance, saying companies should “decide independently” about their operations and deals.

There have been indications Beijing is trying to influence the outcome, after reports this month that officials in China have discussed selling TikTok’s US unit to Elon Musk or using the Tesla chief executive as a broker in a transaction that rescues the platform. TikTok described the report of a potential sale to Musk as “pure fiction”.