Because they have the same life circle as you.
So you can master a certain amount of information.
So when you are with friends, you'd better ask those same friends.
This is the simplest and most labor-saving method. As long as someone has friends, they usually have friends.
If you have friends, you have information clues
If you have a clue, you can connect it with the person you are looking for.
Or you can find someone, or find relatives in China, which is a website dedicated to finding people.
Renren is also good, because all associations are based on real relationships and can expand a lot of information.
There are also QQ alumni records and so on.
There's always a way. Have confidence.
I believe you will find it! Lift your chin! Come on! Self-confidence is the best strength!