Except for a few visa exempt countries, visitors from other countries must obtain a proper visa. There are two administrative regions, respectively Hong Kong and Macau each have their own independent border policy to control their borders and set the requirements for visas.
Other than the above mentioned areas, there are not much restrictions as they are trying to increase tourism. They also want to improve their public image as a nation as well. They have received a bad reputation for so long, but they wish to reverse that. One of these is through tourism.
They even allow people from certain nations to do tourism in specified regions within just 72 or 144 hours so long as they are in transit to another country.