Most balloons are made of rubber, latex, or plastic (mylar).
The plastic balloons contain metal (foil) coated plastic such as polyethylene or nylon. They usually have a shiny, reflective surface.
The latex balloons contain plasticizers – synthetic and chemical compounds which make natural rubber and latex softer and more durable.
Balloons come in different shapes and sizes and are not biodegrade. Therefore, if they end up in our environment, they will take many, many years to decompose (= become microplastics and pollute our planet for decades).