One of the best exercises for learning to draw, if not the best, is to sketch nature.
I use the term "sketch" rather than "draw" as it is more properly a question of "capturing quickly" and not of doing a drawing where you have time to spend on the details. Nothing trains both the eye and the hand as well as capturing reality - ideally that around us - and also making use of the tools available these days, such as photos and videos. So, whether it's from photos, videos, people around you or in the street, make some quick sketches and you'll learn to capture the essence.
On this page, you will see extracts of sketches from my notebooks. These are sketches, not accomplished drawings. Nevertheless, they are of interest in that they capture the essential of what I observe when I do them. Do this type of sketch as often as you can when you have the opportunity.
When we create a drawing, we must remember to give it a posture that is pitched right. What better for this than to observe the postures of the people around us. The same for movements. There's nothing better than studying actual movements for rendering movements well in our drawings.
The same applies to sketches of the face. The important thing is not to achieve respectful portraits, but to capture quickly the essence of the traits and expressions.
Of course, this is not only true for human beings. It's also very interesting to make sketches of animals, which will enrich us through their anatomies and movements that differ from those of humans.