How are the special effects in some disaster films added to the filmed films and how are they synthesized?

Take the day after tomorrow for example.

For example, when the tsunami rushed into the city, the overlooking scene was completely made by computer, including the characters on the screen.

For example, at the end of the film, the professor's friends came to pick them up by helicopter, which was filmed on the set.

Part of the snow on the ground is real, part of it is fiber products, and the background is completely white. Post-production is added to the frozen new york City.

Basically, when the scene is big, it is completely made by computer. Scenes that need actors to perform are shot on the set, and special effects are added in post-production.

If you still don't know much about it, you can search whether you can find the earliest outflow version of X-Men: Wolverine Prequel. This version just hasn't had time for post-processing. You can clearly see that many scenes are completely computer special effects, or the actors are just performing in the studio with a blank background. This is the same.