When we begin playing the piano our hands and brain coordinate to play all the notes roughly at the same volume. At the beginner stage it is not yet possible, say, to play one hand louder or softer than the other. However, with practice you can begin to play the fingers of one hand heavier or lighter. Taking that forward you can gain more control of the fingers and are able to play one finger heavier or lighter than the other fingers of that hand.
The ability to create independence of the hands and fingers takes time and doesn’t come quickly. However as you progress it does become easier and there are some technical exercises available to help.
Ultimately, as your control and touch develops and you can make the phrases ‘sing.’ This is achieved usually by not necessarily playing the phrases or tunes louder, but by having the control to play the accompaniment a little lighter and quieter. You begin to find your ‘voice’ and it opens up a whole new world of expression.
Keep trying, with work and patience it will come!