Space is in the process of being conquered, and it's going relatively well. There are certainly problems, but its stuff all being ironed out, rather than snagging up the works.
Key point, modularity
The Arcology is more than just a megastructure building, it is a foundational technology in the Cosmic era. The structures are in the high 90s in degree of self contain sufficiency, and those that aren't are typically part of an arcoplex, a network of closely related arcos, and what one lacks, another will produce an excess of, creating larger mostly closed systems. What water is in an arco is constantly recycled, and material goods are not just recycled, they are made to be recycled, either through long term use, or designed to be quickly and easily reclaimed for their raw materials.
These buildings are not unique, they are designed to be produced on a larger scale, and while there are local variations, the internal layouts are similar enough that visitors from one arco going to another are unlikely to become lost in the building.
Given the strength required to support a building over a kilometer tall, having it hold a pressurized atmosphere inside is no big deal.
It already has it's own internal power generator in the form of an arc reactor, or dimensional engine, and often having supplemental power from solar panels.
Build it in space, handwave some artificial gravity, and it's a space station or habitat.
A note on artificial gravity: it's a sci-fi trope, and on one hand it is technically lazy on the part of writers and whatnot, and facilitates ease of production for sci-fi shows, on the other hand gravity is one of those topics where realistic sci-fi and traditional sci-fi clash head on. On a personal note, looking at realistic space ships with their axis oriented perpendicular to the engines, large sections of scaffolding, and the occasional rotating ring are so very dull, so very boring. I am fine with artificial gravity because sure, having everyone float zero g through the ship, moving from airlock to airlock is accurate, I have 1 G gravity at my house, and no airlocks. I cannot relate to this sort of environment and I imagine that aside from astronauts, submariners, and a small number of other specialists, most other people cannot either. I'm absolutely fine with spaceships having carpet, and windows, and being laid out in manners that similar to airplanes and naval vessels because these things I am familiar with and can relate to.
back on topic.
Take the arco, change a few bits around on the outside, and add thrust generators on the bottom, and you've built a space ship. It's big, so it can be a cargo hauler, or a cruise liner, or a carrier, or armor it up and it's a warship. There are military fortress type arcos, there are military spacedock stations, it all works out.
Now things get interesting.
The Federation Octohedron type space station is really just two arcos welded together at the base. Mix and match, make the station you need. Attach a medical arco to a residential arco, and you have a space hospital. Attach a space dock/transit unit to a manufacturing arco and you have a spaceship shipwright, attach residential to commercial, space city, and the list goes on.
If you can attach 2, then you can attach four and make a cross.
If you can attach 4, you can attach 6, either flat like a massive 2 km wide snowflake, or one three axes like a jack.
If you can do this, you can add more and more together until the end effect is a rough sphere in shape, made of arcos all cemented together with a common core. Wrap a skin around it, you've got a miniature death star 2-3 km across. At 3 km across, (made of large 1.5 km arcos, or smaller 1 km arcos with a larger core) you end up with an internal volume of 14.14 cubic kilometers, or 14 billion cubic meters of internal space.
Point of inspiration
Looking at the layout of cities, roads, etc from space, at night, the distribution of human civilization follows the same pattern as bacteria growing in a petri dish. What is massive is actually a megacosm of something very very small. The move from cities to arcologies, and habitats is moving away from the bacterial growth pattern and moving towards the structure and growth of something else very very small, but makes very large things, coral.
Arcoplexes and megacities are human scale coral reefs.
The scales are massive, comparing the 344,000 square kilometers of the Great Barrier Reef to the individual coral polyps which are measured in micrometers. Move the scale up from micrometers, to each individual in the collective species being between 1 and 2 meters in size. 14 cubic kilometers is just getting started.