How did stellaris immigrate? What's the use of immigration cards?

First of all, you should allow immigrants in the national policy. Immigration policy does not allow, only mainstream ethnic immigrants are allowed, and immigration is free for three minutes, which can only be implemented if the policy allows.

No manual operation is required. The population will move where they want to go, and everyone wants to go to a place with high livability. In addition, if you want to develop a colonial planet quickly, you can click on the "land of opportunity" in the planetary law of that planet. This will attract unhappy people from other places to settle on this planet.