How does an air bubble have 2 surfaces and a liquid drop has 1 surface?
Think about what a microscopic observer would experience as they moved from somewhere outside the bubble to its centre.
With an air bubble they go from air (outside) to water (the "shell" of the bubble) to air again (centre). So they pass through two surfaces.
With a water drop they go from air (outside) to water (all the way from the surface of the drop to its centre). So they only pass through one surface.
I think there is a little confusion here between a bubble and a membrane.
What is referred here as an air bubble, is actually a membrane of liquid form enclosing a volume of gas, and floating inside another volume of gas (usually air).
And what people refer to as air bubbles, is this membrane, which like any other membrane has two surfaces, while a pure air bubble inside any liquid has only one surface.
A air bubble is hollow inside it. It doesn't pop or pops just due to pressure difference inside it and outside it. Hence it has 2 surfaces- the inside one and outside one
While water drop has only one surface i.e the outside one