Do we know how this character is alive?

Question

NOTE: while i know this is Spyfall Part 1 and the question could be answered in Spyfall Part 2, I only watch the TV series and don't read the books/comics or listen to the audio stories so there may have been a book/audio story that could answer this

in Season 12 Episode 1 Spyfall Part 1, at the end we find out that the O who we've seen so far is actually The Master who shrunk the original O to take his place.

Last we saw The Master it was as Missy and Harold Saxon on a Mondasian Colony ship where Missy stabbed Saxon and as pay back shot Missy in the back with his Laser Screwdriver. while we see Saxon descending down the elevator to his TARDIS and his regeneration process starting to turn him into Missy we know the same wouldn't happen with Missy

MASTER: No. Never. Missy! I will never stand with the Doctor!
MISSY: Yes, my dear, you will.
(So the Master zaps her in the back with his triple barrelled sonic whatever.)
MASTER: Don't bother trying to regenerate. You got the full blast.

Source: chakoteya - The Doctor Falls

now this would indicate that unless Missy was resurrected like when The Master was (or before) Yana he was resurrected by the Time Lords for the Time War or when The Master was Harold Saxon and was resurrected by his followers, The Master is dead-dead.

So do we know how this version of The Master (assuming that this one is one that comes after Missy) is alive?

Answer 1

You've very specifically asked if this question can be answered without seeing part II of this story (which, at the time of writing this, airs in just 2 days anyway). So, here's my best answer based on what we already know.

The Timelords, both good and bad, all apparently wish to "cheat death". The Ninth Doctor called regeneration "cheating death" in Parting of The Ways. The difference between The Doctor and The Master in this respect is that The Doctor has always regenerated ethically - using his own regeneration energy, and apparently obeying the imposed limitation of 12 regenerations, as he was resigned to his death on Trenzalore. The Master, on the other hand, has no such ethics and is prepared to cheat death in any way possible. He has stolen other people's bodies, attempted to steal regeneration energy from The Doctor, been reanimated using the lifeforce from other humans, and of course, he was resurrected by the Timelords - apparently twice (during the Time War to help them fight, and then later he was "cured" by them after the events of The End of Time)

At this point in time (with only part 1 of Spyfall aired) we can only guess at how The Master has come back to life. It could be any of the above methods - or some other way we are yet to find out. Save to say that whatever the explanation, The Master does not stick to the "rule" of 12 regenerations using the Timelord's usual genetic trick.

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Additional information since the airing of Spyfall Part 2 (spoilers):

Spyfall Part 2 did not explicity explain where this generation of The Master comes from. A subsequent episode Prisoner of the Judoon has brought a previously unseen incarnation of The Doctor into the story too, as yet without explanation, which further complicates matters as the new Master could belong to the same time period as the 'new' Doctor. However, Spyfall Part 2 does show that the new Master has visited Gallifrey and destroyed it. The present Doctor then travels to Gallifrey to confirm this. While The Doctor can travel freely through time, the home planet of the Time Lords seems to stand in it's own constant timestream. Whenever the Doctor travels home, time has progressed in a broadly linear fashion. Therefore, it would seem logical that The Master could not have travelled from a different time in the future or the past to destroy the Gallifrey in the present Doctor's timeline, suggesting that he is a current incarnation.