The pen tool in Adobe XD can be a powerful tool for drawing customised vector paths. Simply click to create points to form any shape. By default, all these points start as straight corners, meaning they have no curvature.
To add a curve while drawing, click and drag when placing a new point to begin forming it into a curve. If a point is already created, double-clicking the point will convert it to a curved point and double-clicking again will return it to a square point.