An individual member of the society forms a percept of the world around them. A percept is purely personal. No person can know what percept the other person sees. All they know is how they perceive the world.
Communication, particularly by way of language allows one person to describe their own percept of the world.
A public opinion is formed when people come together in to form of an organised group. A special something is experienced which is far different from their individual opinion. When people transmit their experiences to another by way of communication, their own experience is transmitted in a way that the other person is able to grasp it, by way of understanding how the second person sees the first person. A common, shared experience evolves. In a society, that public opinion exists, which is a cumulative experience of the social individuals.
The individual opinion is never formed in isolation.
Every individual opinion at its core is a public opinion.