pokeanatomy is cool here's a mutilated trubbish and a funny explanation as to what those squishy things are

Trubbish is a hollow pokemon with no skeleton and only a few primitive organs inside some loose, elastic skin. It eats things (mostly garbage), which are expelled into itself through its "open" digestive tract and used as insulation.
Most of a Trubbish's gastrointestinal tract is its mouth, with strong teeth that it uses to cut through hard objects, such as tin cans when eating garbage. When swallowed, the material is filtered through a short amount of intestine, has the nutrients the Pokemon needs to survive drained out of it, and defecated into the hollow space that takes up most of Trubbish. This protects the Pokemon's soft organs and acts as insulation. Its lungs are directly connected to its mouth, with its heart at the center of the body, surrounded by a shell of calcified material.
Its brain is small compared to the rest of its body, and has no skull to protect it, so it relies on the garbage it eats to prevent the brain from getting damaged. Long nerves branch from the brain to its large eyeballs, where a hollow space is made for the eyes to fit in. A nerve cluster in its "ears" is the most sensitive part of the Trubbish's body. It uses this organ not only to hear but to feel vibrations in the ground. It is unknown why it does this.
Parallel to the mouth is a large mass of muscle with "cords" of muscle tissue extending out of it, one directly out of its body, and one to its legs. It rarely uses its legs for locomotion, so it is dependant on its arms to move. It usually directly lines the arm tissue with material it finds.
Trubbish reproduce asexually, first by forming an "egg" out of the material inside of it and a little bit of its own genetic tissue. It passes the egg into a safe place (such as a dumpster), where the baby Trubbish hatches. It usually uses a full dumpster as its nest because when the baby hatches, it already has something to eat and protect itself with.
When Trubbish overeats, it ruptures and becomes a Garbodor. This doesn't phase it, however, due to the amount of material protecting its now-exposed body organs. The Garbodor's "pipes" are just exposed muscle tissue.
Trubbish are usually found in urban areas, preferring garbage due to how solid and common it is. Eating garbage usually allows toxic bacteria to enter Trubbish. The Pokemon and the bacteria then enter a symbiotic relationship; Trubbish gives the bacteria food and a place to stay, and the bacteria protects Trubbish by producing toxins.