This is the most-asked question about China, and it deserves a precise answer rather than a vague one.
What the actual problem is
China blocks a number of foreign services at the network level inside the country: Google (including Gmail, Maps and Drive), Facebook, Instagram, WhatsApp, YouTube and many others.
Whether you use a local SIM or a travel eSIM does not automatically change that. What decides it is where your traffic exits to the internet.
Two kinds of plan
Plans that run directly on a Chinese network. Traffic exits from inside the country and is subject to the same restrictions as a local subscriber. Blocked services stay blocked.
Plans that roam through a foreign operator. Traffic exits through a network outside China. In that case blocked services usually remain reachable, because technically you are a roaming visitor.
Most travel eSIMs sold to international visitors are the second kind. But do not assume — read the plan page, and if it is not clear, ask before you buy.
We will not promise what we do not control
We resell capacity from mobile operators. Whether a particular service is reachable depends on that operator's routing at the moment you use it, and that can change without us being told.
So if your trip depends on reaching Google or WhatsApp, prepare a fallback rather than betting on a data plan.
Fallbacks worth having
Download everything before you fly. Offline maps, tickets, booking references, your hotel address written in Chinese.
Install Chinese apps in advance. Baidu Maps or Amap for navigation, WeChat for messaging. Install and sign in at home — some of them are hard to verify once you are inside the country.
Tell people at home that you may be unreachable on the usual channels.
Email. Gmail is blocked, but some other providers are not. Check yours in advance.
Hong Kong and Macao
Neither sits behind the same filtering system. Google, Facebook and WhatsApp all work normally.
But note: a "mainland China" plan may not include Hong Kong and Macao, because those are three separate coverage areas. If your itinerary covers all three, look for a plan that names all three, or buy separately.