At around minute 57 to 59, Morpheus explains to Neo that every citizen in the Matrix can become an agent like agent Smith, while everything else around them is frozen. How are these kind of scenes recorded/edited?
Combination of CGI crowd, and actual extras holding still. Nothing complex.
Once they hold still, they can use movie editing to freeze some parts of the screen. You would mask them out, only tracking changes in the actors you want.
In other films, just keeping still is common. Dead people are just actors or extras not moving. You can some times still see them breathing.