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. Why did the ban happen? The US government…