china has a billion people. they don't need immigration. japan's population is declining

The solution to people not having enough children is to have more children, you could enlist people from abroad to help with the effort, but you could also try to raise the birthrate using the existing population.

The PRC's total population and working age population will peak in the next five years or so and begin a steady decline from there. 33% of the population will be over 65 by 2050 according to projections.

There was a recent study from Peking University which hypothesized that the population actually peaked in 2018 and is already shrinking. (government numbers are unreliable)

It is estimated to be at 1.4 billion now https://data.worldbank.org/indicator/SP.POP.TOTL?locations=C...

Not just a billion people but millions who are just now entering the workforce

They are not so shut-off to immigration as it is commonly imagined.

Also we might argue that the Chinese "self-migrate" with a huge amount of students that go for studies outside China but return later.

Internal migration is not the same thing as immigration.

It totally is. Someone migrating, say, from Changsha to Guangzhou is going to face a significant language barrier interacting with locals. Yes, they probably both speak passable Mandarin, but possibly with quite different accents affected by Xiang and Cantonese, respectively. An influx of easily recognizable outsiders is going to fuel resentment wherever they compete with locals for scarce resources like jobs and living space. The Chinese household registration system 户口 hùkǒu exists in part to keep that internal migration under control.

Internal migration poses difficulties, but it is not the same thing as immigration. People immigrating to China would probably not speak a language that is mutually comprehensible at all. They would have a much more different culture than internal migrants. But most importantly: immigrants have no claim to being stakeholders in China and how it is governed, whereas each and every citizen does.

You understand that China is a widely different case right?