Individual children follow their individual interests whether or not a teacher is involved. The environment is set up purposefully and with the intent that children be able to engage in meaningful investigation without adult assistance.
Teachers decide when to collaborate with the children in their investigations.
Here is an example of how that can work. A couple of years ago the children in my class developed a relationship with what they thought was one worm in our outdoor classroom. They created small homes and various amusements for Wormy. This went on for months. Meanwhile we were engaging in a series of collaborative investigations, and I chose to stay on the outside of the Wormy project, observing, documenting, but not taking the collaborator's role.