Who or what determines the Alpha Timeline? If Paradox Space determines it, then why are there failed sessions unless those are apart of Paradox Space's "plan"?

The Alpha timeline is the once that leads to the birth of a new universe.  Doomed timelines are supposed to happen, occasionally doomed timelines require some sort of time travel to go back and change the timeline, in theses instances if the doomed timeline didn’t happen then the Alpha timeline wouldn’t be able to happen either.