I've built up two bikes with integrated bars and full internal hose routing, plus just changed to a newer integrated bar on one bike. It's really not that difficult, but it's easier with shift/brake levers that have calipers already hooked up with plenty of excess hose. I cut the hoses at the calipers and insert a short piece of 12 gauge copper wire to prevent fluid leakage. I route the hoses through the bar/stem first, then mount the levers onto the bar, then route the hoses into the frame. Most frames come with thin walled plastic tube pre-installed, so it's not very difficult to route the hose through the upper headset bearing and through the down tube. At the BB, the guide tubing is taped to the hose and pulled in. The front requires no guide. All you do is shove the hose into the hole in the side of the steering tube and push it through. AXS makes the build a lot easier, with no shift cables to install.
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When I changed bars, there was very little excess hose, which makes it tricky to hold the bars up close to the head tube and push the hose into the hole in the steering tube. A helper would be handy, but a short step ladder was my helper.

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