Instant teas are once brewed and extracted into liquid form. Then it is made into a concentrate and dried into powder form.
The good part of instant green tea is that it will immediately dissolve in water with no sediment.
Instant teas will include preservatives such as maltodextrin, citric acid, etc to make it easier to dissolve in water. As mentioned, green tea powder will not have any additives or preservatives, just simply tea leaf for the ingredient.