based on 35m doses being ordered I would guess they will be Pfizer in the end. but generally they could be any dose available.
Pfizer is working on newer ones for the new strains so Id guess these doses will be that.
As i understand it, but could be wrong:
There are no current plans to use anything other than the original vaccines
Trials are already ongoing on Beta specific vaccines (I guess now less relevant given the delta situation)
Trials are planned to start about now on delta specific versions.
Until these trials read out boosters will be with current vaccines.
It might be, just speculating, that a delta specific vaccine gives less good protection than the original against other variants, for instance, and only marginally better protection against delta. In which case we'd stick to the original, I suppose. We'll see.
The current double dose vaccines already give good delta protection and a booster raises it further again.
As an aside, much of this is good news. Despite infecting a significant proportion of the world's population, the virus hasn't mutated enough to escape vaccines against the original. That suggests that we won't need new vaccines against multiple strains every year, in contrast to flu.