A Studio Apartment and an Efficiency Apartment are similar but the main difference between a studio apartment and an efficiency apartment is that the studio apartment is a bit larger in size than efficiency apartments because studio apartments usually come with a larger kitchen with larger appliances such as stove, fridge etc and have a larger counter space.
The bathroom in a studio apartment is usually larger than the bathroom in an efficiency apartment as well.
Both studio apartments and efficiency apartments though have everything in the same room except for the bathroom.
The meaning of an efficiency apartment is an apartment that incorporates the bedroom, living room, kitchen etc all into one room.
It uses a small space to complete the apartment so your efficiency apartment would have the bed, the kitchen, the living room, the dining room all in one space.
Although your bathroom of course would be in a separate room than the other rooms.
That is why the efficiency apartment is called the efficiency apartment because it's efficient to incorporate the bed, kitchen, living room, dining room etc all into one space.
That allows more apartments to be built in the same building.
An efficiency apartment is like a hotel or motel room with everything in one space.