Same as before with the original game: is there a reward if I find all of them? Does it make a difference?
To get to the "star world", like in the original Talos Principle, there is a fan behind dome C. This will blow you across the various small floating islands you see scattered around. Eventually, one of them takes you to a portal for the star world. Accessing it only requires 10 stars, despite there being more than that in the game.
It has seven very difficult puzzles, and collecting the sigil in each of them and then solving a tetromino puzzle below the main platform in the hub world that the other robots stand on will unlock three additional endings.