The robots look like the actual Plankton. Excluding the robot in "Frozen Face-Off," the robots look metallic and have visible screws and metal plates.
Plankton uses the robot as a decoy of himself in the sled race while he goes to the Krusty Krab to steal the formula. However, later on, SpongeBob and the gang finds out about the decoy and that the real Plankton is at the Krusty Krab.
The SpongeBob Movie: Sponge Out of Water
Plankton uses the robot as a plan B so if his tank robot failed, he would switch places with a duplicated robot of himself that would give a fake penny to Mr. Krabs with the real Plankton inside and leave the Krusty Krab sobbing. Later, when Mr. Krabs is gloating, he sees a loose thread on the dummy and realizes that the real Plankton is inside stealing the formula.
Plankton uses the robot as a decoy which goes to Dullsville to retire, while the real Plankton tries to steal the secret formula. Plankton is stopped by Mr. Krabs and SpongeBob because they also sent robot decoys to Dullsville. Meanwhile at Dullsvile, the Plankton Robot, Robot Krabs, and Robot SpongeBob talk about how they were used as decoys. They become friends and have fun with each other until they explode.