I’m assuming this isn’t possible but I thought I’d ask. Those bikes that have no visible cables in the bar area, I assume they must have stem and forks designed for the purpose? I don’t even know how it works – where do the cables go inside? Is it only Di2 that can do this?

Assuming road – bars & forks designed for the purpose.

Various drillings for the bars to get hoses and cables in by the levers, big hole in the middle/back to allow exit into the (normal) stem, then a D-shaped fork steerer to allow cables to run down the front face of the steerer into the headtube.

Or you can do normal bars, special (rare) stem with a clip on cable cover underneath and the D-shape steerer.

The D-shaped steerer is essential I guess, because it wouldn’t work with exit and entry holes..

I bought a 40cm carbon Primavera bar for my Cube 2+ years ago… Settled for just enjoying the extra comfort and ~4cm narrower width at the hoods, rather than go through the hassle of re-doing the hydraulic cables, or paying someone to do it.

One day, someone might sell custom aero road bars with the cable-tidying done by a screw-in cover.