https://www.radiotimes.com/tv/sci-fi/doctor-who-bigeneration-doctorverse-newsupdate/
Diving into said commentary, we hear Davies explain that when David Tennant and Ncuti Gatwa split into two, "a whole timeline bigenerated".
The writer then suggests that each previous regeneration was impacted by the bigeneration, with every 'old' Doctor now surviving his demise in a splinter timeline.
"
I think all of the Doctors came back to life with their individual TARDISes, the gift of the Toymaker, and they're all out there travelling round in what I'm calling a Doctor verse.
"Sylvester McCoy woke up in a drawer, in a morgue, in San Francisco… and Jon Pertwee woke up on the floor of the laboratory," he says.
“Bigeneration, we discover, is an ancient myth of the Time Lords where instead of a new body taking over from the old body, the new body separates from the old body and both are left alive.
"David [Tennant] is parked. For once, we’ve got a happy Doctor who is no longer saving the universe, but is parked with Donna (Catherine Tate) for a happy life, while the Doctor – which is always the next Doctor, and that’s always true of Doctor Who, the Doctor is the next Doctor – is out amongst the stars."