As shared by Jean-Charles Pelayo on Facebook

Neither counting, nor watching the breath in zazen (as when driving a car)
“Sōtō Zen teachers practice and teach in various ways. Each teacher practices his or her own style. Since I am a disciple of Uchiyama Roshi, my own instruction is based on his approach to zazen—neither counting nor watching the breath. It seems to me this is what Dōgen Zenji describes in Eihei-kōroku when he says that inhaling or exhaling are neither long nor short. Whichever practice one follows, it is important to breathe through the nose deeply, smoothly and quietly as the air goes to the tanden (lower part of abdomen) and rises with one’s exhalation.
What does it mean to concentrate if one doesn’t count or watch the breath? Uchiyama Roshi often compared sitting zazen to driving a car. When we drive, it is dangerous to sleep or to be caught up in thinking. It is also dangerous to concentrate one’s mind on an object like the brake pedal, the gas pedal, or the steering wheel. We concentrate our entire body and mind on the whole process of driving a car.
Our sitting is the same. We don’t set our mind on any particular object, visualization, mantra, or even our breath itself. When we just sit, our mind is nowhere and everywhere. Then we can say that our body and mind is concentrated in just sitting. Sitting mindfully, whenever we deviate from upright posture, deep and smooth breathing, awakening and letting go of thought, we just return to the point.”
Shōhaku Okumura, A Path of Just Sitting: Zazen as the Practice of the Bodhisattva Way in Soto Zen, An introduction to zazen, Sotoshu Shumucho, p. 17
Everett Zazen Group
https://everettzazengroup.org/about-2