You misread. Arrow appears in the first episode of Flash and there’s a rumor of a crossover event.

The producers and star have said the want to be part of DCCU but the DCCU brass hasn’t agreed (yet). Constantine wants this too but same issue. The problem is you’re asking for the fanboyish thing I’m warning about “wouldn’t it be cool if these things tied together” which is a lot of hard work to do and do well. As opposed to say the MCU or the DCAU which were built to be connected from the beginning and wrote very well to be cohesive.