The short answer
Separate where you can install from when the plan starts
“Will my eSIM work in China?” can mean two different things: whether the profile can be installed at your current location, and when the provider's paid service period begins. Apple's mainland-China iPhone page says non-mainland carrier eSIMs can't be installed while located in mainland China. The provider separately decides whether a particular package starts at installation, first connection, or another event.
That creates a practical order: confirm the exact iPhone and plan, read the activation policy before touching the install flow, then schedule any required installation while outside mainland China. Do not infer that “install before travel” means every plan's clock waits until arrival.