Miserable Pile Of Words

I know I have examples of this but I cannot for the life of me recall them

Also when does it go from chekov's gun to maguffin?

So in the Inform interactive fiction programming language family's world model, there's a notion of some objects as scenery — objects can be flagged as such (to the best of my recollection) to automatically supply a number of stock responses to IF attempts to verb on them, so that, say, you can have a "sunset" scenery that it's possible to LOOK AT and be furnished a description, but be gently and automatically brushed off as doing something The Game Engine Does Not Allow if you try to TAKE it.

I once read a summary of various peoples' criticism of Niven's Ringworld — a book I haven't read — that, basically, neither the author nor characters really comes to any grip with, nor finds any use for, the titular megastructure beyond strolling round gawping at it for being Really Big.

It is, in a sense, a scenery object. Which I think illustrates that the failure mode of "Wow, cool megastructure" is expecting that to carry the story on its own. Your story's gotta do its thing, whether that's via Chekov's Halo or by establishing cool scenery and then doing something interesting-in-itself with it as scenery.

(I dunno, I'm rambling.)

For something an alien megastructure, I think you can get away with almost completely under-explaining it so long as living in the shadow of something large and unknowable, or completely alien, is thematically relevant. It could also just be something mundane like a corner store, or unfinished, to give an explanation of why it's non-reactive.

For something kinda-sorta similar, I would point to Roadside Picnic. It's not exactly a megastructure, and the stuff sure as heck does things, but I think it still qualifies. Borderline-magical, outright miraculous things are emplaced onto Earth in the intro, and revelation is that there is no higher purpose, they are simply left uncaringly and the interaction with humans is beneath concern.

i read a lot of mystery stories and the form chekov's gun takes is most often is clues about the mystery, but writers often have to go to lengths to describe scenes and clues without giving away everything so its still a mystery by the end, even if the audience is correctly following the trail. a way i see it done often is through misdirection or obfuscation, or polar opposite of singular hyperfixation. that sounds kind of contradictory, but in the book "salvation of a saint" the consulting detective is very fixated on a water filter in the apartment at the scene of the crime as related to the method of the crime, meanwhile the police detectives are more concerned with the motive and social aspects of the crime. both become equally important to the solution, but much more of the book is dedicated to uncovering the motive, while cutting back occasionally to the water filter after it is researched and developed more and eventually becomes the hinge of the entire mystery.
in the context of exploring a setting as a chekov's gun, the same concepts could still be applied, where there are probably more important things going on while being in an alien mega structure, but a character focused on some inconsistency or has a question about the location that can't be answered at the moment, but through the narrative it will be built up and revealed. it'd probably be most effective if it's not contradictory to the other intention of being in the space, like the main mission is "find treasure in the center of the labyrinth" but this one character only has a half translated warning about something from the entrance, but needs to also delve into the labyrinth to gain more data for their translation efforts, and they are still there to aid the original mission as much as possible by reading signs inside the labyrinth. that way the "gun" is not an offhand comment or clue, but something that is building in the background in direct parallel with the main action but it's not entirely focused upon until enough intel is gathered to do something about it.
sorry if this is a lot lol, but i do recommend looking at mystery stories for more ideas since they are the most finely focused version of "set up, build, reveal."

I know this is besides the point, but I cannot help myself:

What you are looking for is Chekhov's gun

Chekov's gun is this thing :)

The City in Blame is barely explained but it influences the entire mood/aesthetic/everything the characters do.

like three different plotlines go on because "the city is trying to kill you," is the operand problem [Blon and the bois hacking, the silicon creatures, Sanakan].

and there was the big response post someone made. intentionality is the main thing.

and the ez way to have it more than just set dressing is like, have stuff/plots/sequences happen that can ONLY happen in a story set in YOUR alien megastructure.

also chekhov was a playwright and a short story writer. with only so many props/words, you better make sure whatever detail you add is there for a reason.

I like the concept of a thematic payoff. don't fire the gun, talk about the gun, why it exists. ideally create some sort of parallel to the real action of the story. the alien megastructure commemorated an interstellar treaty that later fell apart, and the breakup that actually happens in the story feels like the sudden failure of a binary star system, launching stars and planets in all directions. all that remains intact is a wilting vase of flowers in a defunct megastructure, a monument within a monument.