People are always performing. And they have their ways of making you, and others, their audience. Maybe they look at you, or maybe they look away from you, or they point at you, or they put a hand on your shoulder, or they ask you to wait, or tell you to watch or whatever. But they are doing it to get your attention. To keep your attention. They make you their audience.
The simplest version of course is "look at me Mummy!" "Watch this Dad!"
This is the key to who the audience is when you are performing. When you go on stage in character you need to know who the audience would be in the character's life, at that moment. How your character makes people into their audience. Nearly always, consciously or unconsciously, we are performing for someone, recruiting others as part of our "play". Maybe we want a girl to see us, or a boy, or a boss, or an underling, or a friend etc. You get it. We all have our own technique, motivation, style. Some people wait for you to come to them, others get right in your face. Some are subtle, others not so much.
There is of course one more level to this - the fourth wall. If it is "up" then the character's audiences are the other characters on stage and you need to decide which ones are audience and which ones are being used as co-conspirators. If it is down, then the audience is the character's audience, and the other characters (real or imagined) on stage are all co-conspirators. Even this maps onto real life - sometimes we perform and pretend we are not performing, we don't know our intended audience is there. Other times we are happy to "show off" without any pretence that we know they are watching. We might pretend we aren't showing off.
As well as mapping real life onto the stage, we map the stage onto real life. Who is performing and for whom in this scene?
That is the key to how you relate to the audience.