DC All-In is the name for the DC Comics publishing imperative across the DC Comics line intended to launch this October/November and will follow the current Dawn Of DC publishing imperative, intended as a refresher after lockdown and major changes in executives and editors at DC Comics.
As part of that, Absolute Comics will be a line of radically reworked versions of well-known DC Comics characters, from major comics creators, in the fashion of Marvel's Ultimate line, separate in continuity from the rest of DC, but sharing its own internal continuity between the books, unlike Elseworlds, All-Star, or Black Label books. However, unlike the Ultimate line, there is intended from the get-go to be some "bleed through" so that the Absolute Comics line will be in the same multiverse as the standard DC continuity, suggesting awareness of each other, maybe competition or even crossover between them in years to come. It's all part of the plan.
This is not a relaunch or a reboot. DC All-In will continue the continuity of what DC Rebirth did with the New 52, as well as where Dawn Of DC took it. Absolute Comics is a new shared universe from major comics talents, with a hands off approach from showrunner Scott Snyder. I do not know the creative teams. But yes, I presume Scott Snyder will write at least one. Yes, Mark Waid, who is doing Absolute Power, is a natural. And yes, Ram V is coming off Detective Comics just at the time that Absolute Comics and DC All-In hits, so I would look to him, too. But these are just guesses. There are some, like Donny Cates, who were previously attached, but for one reason or another, it didn't work out. But the biggest names, I am sure, are ones I cannot conceive of yet.