Some use something called a “bag-closing chain stitch”, which is done using a special bag-closing machine. The way it works is that the machine punches the thread through the bag material leaving a loop on the back side. It then connects that loop to the next stitch. The thread holds the bag securely closed, but if you pull on the right part of the thread each stitch disconnects in a chain until the whole thing is undone.
Some are sewn shut. If you manage to open them just so the whole thing unravels at once.
Some use something called a “bag-closing chain stitch”, which is done using a special bag-closing machine. The way it works is that the machine punches the thread through the bag material leaving a loop on the back side. It then connects that loop to the next stitch. The thread holds the bag securely closed, but if you pull on the right part of the thread each stitch disconnects in a chain until the whole thing is undone.