The beauty in Star Trek: Unification, a short film by The Roddenberry Archive, is not in the fancy de-aging CGI, but the exposition of a deep friendship that spans space, time, and the multiverse.
My interpretation of this short film is it takes place immediately after Kirk is thrown from the Enterprise B into the Nexus. It's reasonable to assume that the Nexus spans space, time, and universes and thus is able to show Kirk people from throughout his life and ultimately deliver him to Spock in the Kelvinverse.
Their deep connection and friendship is woven through the tapestry of Star Trek itself and their faith that they will find each other again comes up repeatedly throughout their lives. In Star Trek III, after Spock's sacrifice, Kirk risks everything to find a newly reborn Spock on the Genesis planet. In the Regeneration short film, Spock stands over Kirk's grave on Veridian III, hoping to reconnect but not sure how or when. It seems quite appropriate, then, that he didn't seem too surprised to see Kirk at his bedside in Unification as the sun came up.
This on-screen friendship mirrors a lifelong friendship for these two actors; this isn't acting, the emotions are real. Although their relationship became strained at the end of Leonard Nimoy's life, I feel like this film is William Shatner's way of reconnecting with his dear friend.
After Kirk says goodbye to Spock, he returns to the Nexus where a short time later he encounters another captain who asks for his help to "make a difference" and save the lives of millions on Veridian IV. Understanding that he has said a final goodbye to Spock enfuses his actions and sacrifice on Veridian III with new meaning, living up to Spock's edict that "the needs of the many outweigh the needs of the few, or the one".