One word hides many behaviours
Fibre is not one ingredient. It is a broad category of plant materials and related carbohydrates that resist digestion in the small intestine. Inside that category, behaviour varies widely.
Some fibres dissolve. Some remain as plant structure. Some thicken water. Some swell into gel. Some ferment quickly. Some hold water more than they feed microbes. Some feel smooth, gritty, heavy, sticky, or invisible depending on the format. This is why a fibre gram on a label is only the start of the story. For food, beverage, nutrition, and wellness products, the more useful question is what the fibre does in the system where it is used.