Traveling from Shenzhen to Manila with 18hrs layover in Beijing

By the way, I’m a Philippine passport holder.

Ruth, your question is unclear. As you can see, you are talking about a flight:

Shenzhen - Beijing - Manila

This is not a transit through China, but a travel between China and the Philippines with a flight change in China.

I will assume that you will have a Chinese visa since you will start your travel from Shenzhen that is located within China. If I am correct and you will have a Chinese visa, then you are definitely allowed to travel in this way. Simply speaking, your first leg, Shenzhen to Beijing is considered as a domestic one, so you will not exit China until you depart from Beijing to Manila.

To conclude, your segment Shenzhen to Beijing is not yet an international flight and you will not cross the exit immigration in Shenzhen, but in Beijing. So, when you land in Beijing, you will freely and without any immigration formality exit the airport and spend your time in Beijing until your flight departure for the Philippines. Only in that moment will you really exit China and until then you are in China all the time. So, you do not need any 24 hour visa free transit or similar and you will exit the airport without any problem or even without any inspection. Have a nice journey.