
Banana is a sweet tropical fruit with soft, dense flesh, used fresh — muddled, blended or sliced into the drink.
A ripe banana has no true stand-in: pour banana liqueur at half the measure and blend in a spoonful of yogurt to put the body back. It thickens and sweetens at once, and ripeness decides how much of each. Anything from a bottle brings the sweetness without the body, so a blended drink built on fruit comes out thin when you pour it instead.
Plantain
Starchy, low in sugar and inedible raw, and it is a vegetable in practice; no recipe calling for banana means it.
Banana liqueur
Sweet, alcoholic and usually flavored instead of made from fruit: it cannot thicken a drink, which is half of what fresh banana was doing.
A banana ripens after picking, turning starch into sugar: a firm green one comes good at room temperature, and only then is it worth blending.
Room temperature, out of the sun, and away from other fruit unless you want everything ripe at once. The fridge blackens the skin within hours and leaves the flesh perfectly good, which looks alarming and is not. Peeled and frozen in chunks is how to keep a glut.