Then how does the split of alt histories occur? Trunks is in a future where the og zamasu meets black before zamasu can even kill gowasu or is himself killed. That black came from a timeline where he was not killed by beerus, obviously, yet he did meet and fight goku. This implies that Black is from a third timeline, beyond that of Trunks and 'our' one, in which he he meets goku does but does not get killed by beerus (whether black kills gowasu from his timeline is unknwn) and travels to Trunks'. Three timelines, with three different outcomes after zamasu meets Goku (which has to take place in each one).
The problem is, black knows goku BECAUSE goku goes to zamasu, BECAUSE trunks warns them of black. In Blacks original timeline THAT trunks is then from ANOTHER timeline. This means that the Trunks from black's original timeline comes from another future with its own timeline, and its own Black. In other words: zamasu creates black BASED on goku meeting him, which is itself based on an event from another timeline where a trunks meets a black. Black's creation is based on events from two timelines, one in which he does not exist, but is created, and the other in which he meets trunks. (And in the third we see, 'our one' he's killed.) That's like a camera filming its own signal on a tv, it goes on into infinity. Yet now a timeline occurs where zamasu is killed. So even weirder, somewhere in that infinite feedback loop is possibly a different image from the rest: our timeline, where Beerus kills zamasu. It's like 'infinity+1'. Unless zamasu is restored in our timeline, black IS created in our timeline, then it 'just' infinity, lol.
Either I'm missing something, or this is SO weird, I kinda like it.