Stay in a hotel in China and this is handled for you at check-in. Stay in an apartment, a friend’s spare room, or anything booked through a short-let platform, and it becomes your responsibility.
It is easy to miss, because nothing in the booking process mentions it.
The rule
Foreigners staying in private homes must register their accommodation within 24 hours of arrival.
Failure to register can result in a warning and a fine.
You can now do it online, in some places
Online registration is available and carries the same legal effect as registering in person, so it is not a lesser option.
It can be done through the National Immigration Administration website, the NIA 12367 app, the NIA 12367 WeChat mini program, or through Alipay.
The important limitation: online registration is a pilot, currently running in seven provincial-level regions including Hebei, Liaoning and Zhejiang. Outside the pilot areas the in-person route still applies, so check before you assume the app will do.
Your host is part of this
The first registration should be completed with the help of whoever owns or manages the accommodation. They hold details you will not have, and in practice they have usually done it before.
If you are booking a private stay, it is a fair question to ask the host in advance rather than on arrival.
When you do not need to repeat it
You do not need to register again for a repeat stay at the same residence if you have registered there before.
Foreigners returning to their own residence in China after traveling do not need to re-register either.