1. You absolutely aren't allowed to take your regular phone and laptop;
2. You will be given loaner devices to take into China;
3. If you're asked to open such devices on entry, comply and then, when you can, inform IT;
4. Once you got back, I'm not sure what happened to those devices. I believe they were in the very least wiped. They may even have been destroyed in certain circumstances (eg if a border official examined the open device). But that's speculation.
I never travelled to China so never used this. A colleague who regularly traveled to China told me some stories about this.
But yes it does seem prudent to wipe your device and restore when you land. Then again, border officials can also deny you entry with very little justification so who knows?