As Saddlesaur said, lots of various ways have been used for routing through frames. I’ve been unimpressed with pretty much all methods other than what is on my '20 Trek Domane SLR. It uses full housing all the way, AND has an access door for storage in the downtube of the frame which also just happens to provide a way to see your housings and hoses going through and manipulate them from that point, midway down the downtube.
@ozmadman said he had the thin guide tubing through the frame when he built the bike. That’s critical to save (or get some from a shop) for some frames because you’ll use it every time you run another cable in that frame. It lets you run that guide in through the frame ON the EXISTING, OLD cable before you pull that old cable out, leaving the inner guide in place and sticking out at both ends of where it runs through the frame. You then can use that to guide your new cable in through the shifter, the first section of housing, and then the frame and then just pull the thin guide housing out once you have your cable through to whatever is the last internal section of the frame. This is what is needed on my Trek FuelEX, and makes the task at least pretty manageable.
I’m not a fan of frames that have a permanent internal tube for each cable (or hose) even though it can make cable changes a lot easier than fiddling with those removable thin guide tubes. My biggest concern with that design is what is required if the internals fail, which is probably rare.
The only downside I see with running full housings internally all the way through is the potential for noisy rattles if they are not secured well, and that can be a real problem. My Domane has channels that clamp onto the housings/hoses as they run through the downtube, holding them snugly in line. That clamping bracket is mounted to the carbon frame, and is accessible through the storage access door in the downtube. The housings run through the BB area and then the chainstays to derailleurs and rear brake caliper, and are secured where they terminate. They are very quiet and don’t rattle around. (I have not changed those housings yet, so I might learn something new in the process, probably later this year. Saddlesaur’s description of running the new one in, butted up against the old, with a cable running through them is roughly what I expect to do.)
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