The "Robot-building device" is the machine brought and modified by Phineas and Ferb Fletcher in order to make the Brobots. This one had two versions; the first one, which was pretty complicated, and the second one, made by Phineas and Ferb and easier to use.
The first version is divided in several pieces, one of them being a monitor, and some more being some kind of potions, along with a lot of other machinery and complicated stuff.
Second VersionThe second version, as mentioned above, was made by Phineas and Ferb, and is also far easier to use than the last version.
This version looks like an enclosed with a photo of a recycle symbol on the door and a red alarm on top of it, which flashes red as it makes the Brobots.
Unlike the Phinedroids and Ferbots, the machine was never really destroyed or taken away, but it was not seen in any other episodes.
However, it and a few copies of the machine were seen in "Phineas and Ferb The Movie: Across the 2nd Dimension", where it is used to bring the Phinedroids and Ferbots to fight in the battle against the Norm Bots and Heinz Doofenshmirtz from the 2nd Dimension.