This is all extremely unlikely unless you are a known enemy of the state or are bringing in illegal goods.

In customs they simply ask you why you're visiting, where you came from, if you're bringing any illegal goods, then let you pass through. If you're flying in, there's usually a huge line of people at the airport and you only end up talking to the customs agent for 10 to 30 seconds.

If you have a very suspicious story that doesn't check out or you're trying to bring in illegal goods to the country or you know that you are doing something nefarious then you should be concerned that they might pull you into a back room. I think all of these concerns are very overblown for the average person though.

When I was reading about whether to bring my phone to China, everyone online had similar recommendations about bringing a new device and it ended up being completely unnecessary. They don't check anyone's phone and they asked no questions other than reason for visiting. When I told my co-workers about my concerns they all thought it was quite comical because they've traveled more than anyone and see this as extreme paranoia.

Of course they could take your phone and put you in jail for any reason they want if they really wanted to, the most important thing is to not give them a reason to. They will only get your phone if you give them a reason in the first place.

Just keep your phone locked/off when you go through the border so it requires a pin.