Modern shooting technique: the actor sits in front of the green cloth and performs normally. Then, they use the computer to type and join the crowd in the foreground and background, and speed up the playback when editing. This effect can also be achieved.

This technique is generally used to show that the characters in the film are lonely or independent at that time, and the people and things around them have nothing to do with them.

I hope you will be satisfied with my answer.