This is a common question asked by many who attempt to buy a punching bag, especially online. To save on shipping costs, which could be gigantic, when talking about the 150 pounds (70 kg) punching bag.
Long gone are the sandbags and bags of sawdust filled. It was only back a solution on the day and resulted in many injuries (mainly wound wrist) due to the fact that the punching bag that is as hard as concrete. Nowadays it filled with different types of material. You can “buy a punching bag” (which is known as “boksbal kopen via https://everlastonline.eu/nl/boksen/bokszakken.html” in dutch).
Times have changed and today people use gloves that are smaller and suitable. With the ever-growing world kicking MMA, there are plenty of MMA gloves on the market that is suitable for any type of boxing and training.
Today's bag separate from normal punching bags and the so-called "heavy" bags. The ladder is still close to the weight of the old fashioned ones but softer to the touch none the less. Most of the manufacturers use cotton rags (shredded textiles) to fill the bags with. This gives it the right volume, weight, and hardness.
Some providers (especially Chinese) use a component of shredded rubber, which they get from shredding used tires. This is convenient and easy to fill up the bag with but lacks the softness and usually ends up settling on the bottom and hardening the bottom part of the bag.