What is the probability that HKU will immigrate to the United States after graduation?

You have two events here, one is to go to the United States to study for a doctorate, and the other is to immigrate after you go.

After finishing your doctorate, it depends entirely on your research direction and the articles published during your doctorate. After all, raising a doctor is all funded by the professor himself. After professors without funds cannot improve their doctoral degrees, professors with funds have a high demand for doctoral articles, so the probability depends on their own situation.

As for immigrants, you can get EB 1A in one month, and after 5-8 years.