Why did the US just ban TikTok from government-issued cellphones?

Trump tried to impose a total ban on the China-based app and some states have already prohibited its use on official devices

The US government has approved an unprecedented ban on the use of TikTok on federal government devices. The restrictions - tucked into a spending bill just days before it was passed by Congress, and signed by Joe Biden on Thursday - add to growing uncertainty about the app's future in the US amid a crackdown from state and federal lawmakers.

Officials say the ban is necessary due to national security concerns about the China-based owner of the app, ByteDance. But it also leaves many questions unanswered. Here's what you need to know.