How do i paint with brushes in Adobe illustrator
Brushes aren't really suitable for painting in Illustrator. That's Photoshop. Or Fresco
To explain this: what is possible with Illustrator brushes: they can mimic oil paint strokes or watercolor strokes. Single strokes. They won't behave like oil paint and mix with the strokes below them or dissolve into the paper like watercolor does. The strokes can be transparent and thus somewhat interact with what is below them, but they won't change it in behaviour, like pixel based painting apps can.
Tutorial: Watercolor https://www.youtube.com/watch?v=x7SnN4mfYEQ
Brushes that mimic oil paint: https://www.retrosupply.co/products/dragstrip-2-vector-brush-pack (using those brushes and creating that look is more like drawing, overlaying shapes and constructing. It has nothing to do with painting.
Of course, you can paint/draw with brushes in Illustrator. You would have to create suitable custom brushes that may satisfy your demands.
Drawing is something else than painting.
The term "Painting" the way I understand it (and see it used among illustrators) means mimicking analog media such as watercolor, acrylic or oil. Yes, there are brushes that look like oil paint strokes. Trying to use them in order to paint an oil painting will fail.