Hi Nicoloc,
On the TT-X21, and other internal frames/handlebars I guess, the cables/hoses go straight through the bars, stem, head tube, and into the rest of the frame without ever exiting the bike. This is my first bike build, as well, and on the TT-X21 as far as I know, you have to run the cable housing all the way through, one piece. There are no guides inside the frame for cables or the housing. I added some housing dampener and everything seems to be working fine.
Clarification, the rear derailleur cable housing runs all the way from the shifter, into the handlebars, stem, and frame, then exits the bike at the rear, running all the way to the derailleur in one piece. I didn't see another way to run cables to the RD by using the short piece of housing that comes with the rear derailleur.