This is the second adaptation of "Crisis on Infinite Earths" as it was previously adapted as a crossover between the CW's DC shows as Crisis on Infinite Earths: Part One (2019), Crisis on Infinite Earths: Part Two (2019), Crisis on Infinite Earths: Part Three (2019), Crisis on Infinite Earths: Part Four (2020) and Crisis on Infinite Earths: Part Five (2020).
The Anti-Monitor takes a form completely different from his comic and live action counterparts; being a jet black being, rather than be a living armor or some kind of shriveled pale being.
It's not explained how the Monitor, nor the Flash by extension for creating the new timeline, allowed the Anti-Monitor access to the multiverse.
It makes sense Terry calls out the Tommorowverse Batman on refusing help and choosing to be alone. His Batman had a Bat Family and only ever let them go after extreme circumstances; Nightwing couldn't handle Bruce hurting criminals in front of their children, Robin was driven insane and experimented on by Joker, Barbara suffered a miscarriage while crime fighting and broke up with Bruce. At least in his old age, Bruce had a dog and accepted Terry's company as the next Batman.
Constantine managing to get his memory sorted out and his old witty persona back is major highlight. The man had been suffering millennia all because the universe was ungrateful towards him and the Flash trying to undo the damage Darkseid did.