Development Hardware
This page will contain mostly theories about how Super Mario Galaxy specifically may have gone from development to release, given our current knowledge about the real physical and strategic components and processes we do know developers had used.
Various hardware for developing and testing games/game builds was made available to 3rd Party developers by Nintendo and also 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 sometimes/internally by Nintendo at that time as "MarioGalaxy/Super Mario Revolution", to name a few) was being developed early alongside the Wii's development too, then 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 (development) 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, and due to the NDEV's lack of an optical disc drive; disc images would have to be emulated on a PC Host streaming to a NDEV, with the host running the needed software provided in the Wii/Revolution SDK(s). Given this, there are chances that those digital debug/beta builds of the game, or even the renowned 2006 E3 Demo existed or still exist, depending on how Nintendo treats their old 'classified' game files and software.
It is also possible the 2006 E3 Demo of the game was played on 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, instead of wasting a RVT-R Disc, which this unit was great for developers testing early builds of their games on) 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 (and especially close retail builds) on disc media, as would the consumer would do with their retail Wii using retail release Wii game discs.
Given the nature of how a RVT-H Reader is able to play its games, many who have been lucky enough to find and buy one of these units online in places like Ebay, have been able to find disc images left in their Hard Drives, likely by a previous owner or developer who once worked with that unit and maybe forgot to remove any digital content on it. Many previously undumped or even completely unknown games/game builds and software have been dumped from these units and properly preserved this way.
For writing data to the various mediums mentioned in this page, GameCube game 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 a NPDP Reader, developers would have used a NPDP Writer to write a disc image to a NPDP Cartridge. For a RVT-R Reader, developers would have used a RVT-R Writer to burn a disc image to a RVT-R Disc. It is also believed the RVT-R Writer is compatible with NR Discs, assumed by the presence of a Mini DVD sized area within the main DVD-R sized bay. Although that has not been confirmed, given that a RVT-R Reader is backwards compatible with a NR Disc, similar to how a retail Wii (if it is the RVL-001/GameCube backwards compatible model) is backwards compatible with a retail GameCube disc, then this may help to prove that theory.
Once a game build is ready to be tested for retail release, a developer would write/burn a copy of that game disc image to a NR Disc or RVT-R Disc to then be handed over to their playtesters/debuggers. If they were happy with a build they played through and tested, a developer would then send that game build in to Nintendo for lotcheck, a process where Nintendo, given their tools and software, would further test the game build for issues like code vulnerabilities, bugs, ensuring proper formatting, etc., before it is ready to be handed over to their disc pressing factories where it is finally pressed to become a retail GameCube or Wii disc.