Tinder and to a lesser extent Tantan.
I would venture to guess like 90% of the African and Vietnamese women using Tinder here in Guangzhou seem to be working girls. Some have offered me one shot full service for as low as 500 RMB, which ends up being more when taxis fees are involved if they come to you (or if you have lots of free time and want to explore the metro, you can arrange to go to their place). Most seemed to charge about 1000-1200. In my experience, they don't seem to bring or have condoms ready, so bring your own bag.
The escort business cards they put under your hotel door or toss out on sidewalks.
For the most part you're supposed to call or text the number and they send someone to your hotel room or apartment, but you never know who will show up to your place.
However, some have WeChat QR codes, or you can simply try searching the phone number's WeChat account and sending a friend request. You will usually get an agent that will post the pictures of the available girls in his or her WeChat moments, which are arranged by location, or who will send you a different QR code that sends you to a website that basically lists all their available technicians, or who will WeChat message you to ask what type of girl you are looking for and send you a bunch a pics and you choose which one you want to see. Each listing is a menu with pics, vids, services provided, extras, height, weight, age, bra size. Either way, you tell the agent which girl you want to see and at what time, and he arranges it with girl and tells you where her incall is and you go to the service provider's incall. The incall is usually but not always a hotel or an apartment Tancun if you choose a girl in the Tianhe District, or a hotel on Guangzhou Dadao if you choose a girl in Haizhou, or one of the hotels along the river if you choose a girl in Yuexiu. I have no idea where the Baiyun or Panyu incalls are.
The service is expensive for China. The least expensive listing I have seen was 1000, and that was a one off special. They are usually listed as somewhere between 1500-3000, but probably more because most of the agents want a higher commission than that, and most of the girls, whether Chinese or foreign, ask for extra to service a foreigner. You can try haggling with the agent over the price beforehand. Sometimes they will budge, sometimes they won't. So I get it, it's expensive. However, if money isn't an issue, the benefits are it is full service, 90 minutes, two shots on goal, massage, shower, in a clean room, on a bed, the girl matches (or mostly matches, there is a lot of filtering in the picture apps they use) the picture, and the listings usually include a brief video clip as well to further assure you what you see is what you get. Also, since there are several girls in the same hotel, you can contact the agent and ask to see another girl if the girl who opens the door was not what you were expecting at all. Generally speaking, the Chinese techs will wash you when you shower and give an uncovered blowjob, while the foreign ones rarely do either of these, but your mileage may vary.