Technically, pure mental concentration is samadhi, since dharana has object, and without object it is merely of the nature of samadhi. Dhyana in itself is not flow, and the “athlete’s flow” is prior to even the first dhyana, but it works as a worldly example. In fact, dhyana is the state of unity, and samadhi is clasically defined as mental absorption, and ultimately, dhyana is also of the nature of samadhi. The dhyanas are the form realm samadhis.
Samadhi is exactly and ultimately focusing your mind on its own nature, and the “all”, oneness, is an appearance that occurs at the higher level samadhis and dissapears at the even higher level ones. The expanse of oneness is no different from the nature of your own mind, and the expanse, the feeling of oneness, and the idea of a mind, expanse, or samadhi, all melt in samadhi.
Thank you for the comment.