There is many different banana types, all of them are similar in taste and sweet when ripe. Green banana is just not ripe yellow banana. Must be cooked or fried. It’s is not sweet… taste good for example in: “Green Banana Cakes”. Green bananas are called “figs” in some parts of the World, more precisely at Dominica in the Caribbean.
There are also plantains, they have green skin that turns yellow and then brown and almost black when they are very ripe.
They look like bananas but are bigger and more shapeless. Ripe plantains have sweet taste but must be cooked or fried before consuming, for example “Fried Plantains”. Green plantains are not sweet and can be used like green bananas, cooked or fried, for instance in “Salt Fish in Coconut Milk” or “Vegetarian Mofongo”.