Tubes come in plastic (nylon), metal (aluminum, brass, steel), and are either cut or pre-cut to length.
They require a fixture that clamps into the vise to hold them while tying.
The leader is passed thru the tube and tied on the other end, the hook is held in position using a short segment of flexible tubing, preferably silicone. This is available in various colors to accent the fly itself.
For river fishing applications, most prefer the short shanked beak style hooks, like egg hooks but much bigger - straight eye. Partridge Nordics are a good example.
When a hook is damaged, the tube fly is unaffected.
They eliminate long hook shank leverage problems when fighting a hooked fish because the hook is small and un-attached to the shank. Basically a little lip-ring even on huge flies.
You can pop them into a shirt pocket without piercing your nipples.
I use a variety from plastic ocean streamers for coho salmon to steelhead / salmon flies to striper squids. There are really no limits.