Transit

The first and the most important detail is that there is no such a thing as 72 hours transit visa. Since your layover is shorter than 24 hours, you will be definitely eligible for 24 hour visa free transit, but keep in mind that there is no visa at all of any kind. In other words, it is your right to travel visa free through Shenzhen. If your layover way longer than 24 hours you would have been eligible for 144 hour visa free transit. So, 72 hour visa free transit was replaced in Guangdong province (where Shenzhen is) with 144 hour visa free transit years ago. In any case, you travel under 24 hour visa free transit.

However, there is another detail... The above does not imply the right to leave the airport, but just to travel visa free. In order to leave the airport and explore the city and the surroundings or have an overnight, you will approach, immediately upon landing in Shenzhen, to 24/144 hour visa free transit counter where you will apply for a temporary entry permit. This application is for free and here I suggest you to book your accommodation in Shenzhen in advance and to have your accommodation confirmation in hard copy. You will present it to the authorities and based on their judgement, you will be granted a permit to exit the airport. The issuing of the permit always depends on discretionary decision of airport authorities, but having in mind your long layover and if you book some simple accommodation, your chances to be granted a permit are practically almost certain.