This is cool. Would like to see it hit Steve's sights

If it were immobile, you'd expect to find old examples spread throughout the galaxy, remnants of older spacefaring civs, on both planets and asteroids. Asteroidal examples would likely be associated with TN mineral deposits, although likely only small and/or low acc deposits, the last uneconomic remnants of a once better deposit, and on planets mostly associated with ruins, although you might also find mysterious UI sites with no apparent abovegound component too... The traces of a city so ancient that nothing remains, or evidence of a Great Scouring, with all visible signs of civilisation having been destroyed by spoilers? Woooooo, spoooooooooooooky, WOOOOOOOOOooooooooooOOOOOOO

That... would be interesting, yes.
Ancient ruins, but simplified.

Of course, I prefer if these underground remnants spawn only, if you turn on certain spoilers.
Since, some may prefer to play as if humans are the first civilization to have made it to space.

I think the idea is interesting. But it should not be transportable so you would have to build it on site.. digging out those asteroids with construction brigades.

This is perfect. Make it cost the same as normal infrastructure but either make it twice as effective, or function as LG infra, or even both. In the long run it is more expensive as it cannot be reused on another body but in the short term it is cheap and simplifies logistics.

I would actually really like this as another option for asteroid mining since it lets you repurpose manual mines more easily during the early mineral crunch in Sol as trying to keep up with infra shipments by freighters is tedious and frankly gets expensive.

 - I'd be interested in seeing the return of underground infrastructure. Specifically as kind of infrastructure that not only functioned like Low-Gravity Infrastructure, but was twice as effective as normal infrastructure, AKA it counts for two Infrastructure. It would cost 15 BP, 5x Duranium, 5x Boronide, and 5x Mercassium. Additionally, Underground Infrastructure would be able to increase a planet's total population up to the body's max population, but also increase the density by the same percentage on top of the additional population's increase.

 - This would allow player's to create an Ecumenopolis like Coruscant. I'd suggest a tech line that tied the increase to a percentage, capping out at 100% and starting at around 10~30%, so it'd be more meaningful as a choice. This would also perhaps allow an NPR AI to focus around it.