Hi how do I paint inside a shape to add texture like you can in sketch? Like on the trousers here
Use the selection tool to select the pants and then paint inside of the selection. The selection tool lets you draw the shape with a polygon lasso or paint the selection with a brush.
You can lock the transparency of a layer which allows you to “paint inside” anything on that layer. If the trousers are on their own layer it will work. If the entire illustration is on a single layer, you need to use the selection tool to protect the other areas while you paint. I hope this makes sense.