In 1570 on the first day of the Wildy moonslice, the Gods held a QnA in Mancanllamaland, a small town in the Land of Subsolar Sands (small is an understatement, Mancanllamaland has just 56 people). In this QnA the Gods spilled the secret about what is below the Bedrock Platform.
Below the world is just the Axolotl God's hangout area, and the Playground's maintenance place. Gravity faces towards the bedrock platform so they basically have a whole world for just them to keep the top side running.
They say it's landscape is a giant barren rock landscape that, due to a lack of water (actually there is water, just much less, only enough for a small sea), the infinite frozen wastes is not icy like it is on the other side, they're just really fuckin' cold and a coated in a thin layer of frost. It is not dark, due to a copy of the Sun and Moon used for maintenance.
Some things they explicitly mention:
A hangout area with games and snacks.
The one place to edit the Gods' Magic Script, the same as the one people can change but because the only way to access it is down there, its de facto only for the Gods.
A copy Sun and Moon in accurate positions with accurate colors and what they are doing.
A way to keep an eye on everything, they are Omnipotent, but not Omniscient, so they use the God's magic script.
A way to copy a section of existing surface ecosystem for testing new species.
A planning and designing and coordination room.
They also said that most quakes are caused by tests on the other side of the platform, that neither side is the top or bottom (they refer to each as just "Playground's Side" and "Gods' Side"). This is also where they activate the Special Hours.
They said there is in fact a small unique ecosystem that sprung up 430 years ago when some life escape one of the tests, when they copied a section of ecosystem from a tundra on the Playground side, some lichen, moss, mites, ants, and flies escaped the test area and got established across the Gods' side, Goldy tried to use fire to exterminate the lifeforms because having living things running amok in the place the Gods maintain the universe could interfere with testing, but they failed. Now they just have designated testing areas that are regularly incinerated and sterilized, and their maintenance areas are kept clean as well.