Unconfirmed but I believe that these are (via Den of Geek):
Stripe Robot (rear)
Luke Wilson will play Courtney's stepfather, crimefighting partner, and former superhero sidekick Pat Dugan, the pilot of the STRIPE robot. Here's the official character description...
“Pat Dugan once went by the less-than-inspired superhero name of Stripesy, ...
Earth 38: Supergirl
Might be a nod to the debut of Superman (1938) as mentioned in another answer.
Earth 16: Future Oliver
The debut episode of future Oliver was in Legends of Tomorrow during first season (2016)
Earth-99: Future evil Batman
Might be a nod to DCAU (Batman Beyond premiered in 1999)
Earth-75 Dead Superman
Issue #75 when Doomsday killed ...
After watching the crisis episodes, IMO I guess you have to definitely watch Arrow and maybe Flash and Supergirl.
ARROW: (Important to watch)
The final season deals with the build-up to the crisis event and also throughout the season hints at
It also explains more about the crisis' effects and about the Monitor and so this season of Arrow is important