Stand up pouches are filled using automated filling machines. The human role is limited to keeping an eye on these machines as they do their job. Although each machine has a slightly different method of packaging food inside stand up bags, the basic principle stays the same.
A bunch of empty food pouches is placed in a container. A mechanical arm picks up one plastic pouch at a time and places it on a moving belt where a pair of moving arms opens the mouth of the pouch, readying it to accept liquid or paste from a nozzle. The nozzle is programmed to eject only a specified amount of food. When the food product has been dropped into the pouch, it is moved to a sealing machine which heat seals the mouth of the pouch. All this happens without human intervention.
As mentioned above, the exact details of how food is packaged in a food pouch may differ slightly in different machines but the essential principle remains as described above. For instance, a machine packaging potato chips in simple plastic pouches works differently than a machine being used to package juice in spouted pouches. The first machine needs to be equipped with tools to pump nitrogen in the pouch, while there is no need for that in the second machine.