MichaelW

There are a variety if different styles of touring bike:
Light
Medium
Expedition
Light touring bikes are designed for day rides and fast supported/hostel touring. They are often made of racing materials but with clearance for medium tyres and fenders and a small luggage rack. Gearing is usually compact double for road triple.

Med tourers are capable of short camping trips so are made of heavier duty materials with more tyre clearance ( so require cantelever brakes), 2/3 waterbottles, front+rear racks. They are agile enough to be rideable unladen as commuter bikes and capable off-roaders. Gearing is road or MTB triple.

Expedition bikes are purpose-built for long term camping style trips on rough roads. They have loads of tyre clearance, often for 26" MTB wheels and all the braze-ons. gearing is MTB triple or Rohloff hub gear.

The geometry of the bike is tuned to the expected load, heavier loads needing more stable steering. The rider position is a matter of choice. Many have a slacker seat-tube and higher head-tube/handlebars. The chainstays are long to place the load inside the wheelbase and to avoid heel-clip on the panniers. Bottom brackets may be low to lower the centre of gravity.
M and exp bikes use the MTB standard (135mm) for rear dropout spacing instead of road (130mm). This permits a stronger wheel with less dish but you cannot use boutique wheels.
Material of choice is usually steel, it is tough, reliable and well proven with tubesets tuned to touring use over many decades of experience. It is reasonably cheap, "easy" to work in a small workshop, brazeons can be adapted as required. You can get touring bikes made from Al or Ti but so far, not in carbon.