So the person who was building was planting bricks and at the same time moving the ghost brick?
Answer: They are pressing the "plant brick" and directional movement for the ghost brick at the same time. All keyboards can do this.
Or were they just suddenly creating large multiple-brick things?
Answer: They are using the Duplicator Add-On tool to rapidly recreate sections of bricks over and over again.
Or were they just building really fast, too fast for a normal person?
Answer: They are using a built-in feature of Blockland called Macros. Creating Macros is pretty tricky and requires a lot of practice to do it right.
