Development Hardware
Various hardware for developing and testing games/game builds was made available to 3rd Party developers by Nintendo and even presumably used by Nintendo themselves. For the GameCube, these were mostly units with model numbers starting with "DOT" instead of the retail "DOL". For the Wii, these were mostly units with model numbers starting with "RVT" instead of the retail "RVL". It is believed the "T" in these model numbers stands for "Tool" (Ex. DOT = Dolphin Tool (Dolphin being the GameCube development codename), RVT = Revolution Tool (Revolution being the Wii development codename)).
Since Super Mario Galaxy (referenced internally by Nintendo at the time as "MarioGalaxy/Super Mario Revolution") was being developed early alongside the Wii's development as well, there is a chance physical media exists or once existed for the game on either a NR Disc or a NPDP Cartridge and was thus playable on a NR Reader or a NPDP Reader; possibly the only official instance in which the game was playable in the GameCube assembly line of hardware.
Later into the game's development, and as the Wii also neared its more final, retail form, further development and testing of the game likely took place on the main Wii development units, the NDEV (the name likely standing for "NNGC Development", further likely standing for "Nintendo Next Generation Console", with references to this name in the various Wii/Revolution SDKs) with possibly dozens of digital debug/beta builds of the game built, due to the NDEV's lack of an optical disc drive; as disc images would have to be emulated through the NDEV using provided software in the Wii/Revolution SDK(s). Given this, there are chances that those debug/beta builds of the game, or even the renowned 2006 E3 Demo existed or still exist.
It is also possible the 2006 E3 Demo of the game was played on either a RVT-H Reader (which would have been easier for Nintendo to set up since they could have just simply copied the needed files to the internal HDD) or on a RVT-R Reader burned to an optical RVT-R Disc; with the latter being the more probable, as it's more economic to test showcase (or especially close retail builds) on disc media, as would the consumer would do with their retail Wii.
In the GameCube development hardware era, developers would have used a NR Disc Writer to burn a disc image to a NR Disc for it to have been playable on a NR Reader. For the RVT-R Reader, developers would have used a RVT-R Writer to burn a disc image to a RVT-R Disc. It is believed the RVT-R Writer is also compatible with NR Discs, assumed by the presence of a Mini DVD sized area within the main DVD-R sized bay- although this has not been confirmed. For the NPDP Cartridges, developers would have used a NPDP Writer.