For Google they talk about how the same search can give you different results depending on the user, so for instance they mention search about "climate change" it can return you first neagtionism or confirmation news, based on what Google estimates that will make you to engage more, not by which one is the most relevant / accurate / true

It doesn't have to be a social network. They track how you use their apps and software and construct them with the goal of keeping you in their ecosystem as much as possible. Tristan Harris, one of the main people in the movie, worked on GMail when he started wondering why they weren't focusing on helping people use their apps in a more responsible, less addictive manner.