Yes it's hard, which I acknowledged by saying if they have ambitions, go big and recycle materials in space. But you make it sounds like it's nearly impossible, which I doubt.

We know how to keep air in space stations and capsules, without involving force fields or any other sci-fi tech.

For sure, building in space it different from building in earth gravity, but that doesn't necessarily make it impossible. There already have been experiments and small-scale demonstrations in space:

In 2014, the NASA, in partnership with Made In Space, Inc., demonstrated a fused deposition modelling on the International Space Station and published a summary of zero-gravity experiments in 2019.

Another example is a microgravity extrusion experiment in the ISS between from 2021 to 2023,

I assume it's easier to start by building small parts, and progressively build larger parts, until hopefully we're able to build most ships parts. The assembly can presumably happen in the vacuum of space, without air. There's potential for ultimately building ships in orbit larger than anything we could lift with a rocket.