There are some cross-overs Dem.
Okay..
FLASH S2 EP8 concludes in ARROW S4 EP8
SUPERGIRL S2 EP8 sets up the 'Invasion' story that begins in FLASH S3 EP8 then ARROW S5 EP8 then LEGENDS OF TOMORROW S2 EP7.
SUPERGIRL S2 EP16 sets up FLASH S3 EP17 musical episode.
SUPERGIRL S3 EP8, ARROW S6 EP8, LEGENDS S3 EP8 is the 'Crisis On Earth-X' crossover.
Other than the random mentions of characters from other shows these are the main cross-over arcs.
The 'Invasion' cross-over is available separate on DVD if you don't watch LEGENDS.
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I Expect the same will be done for 'CRISIS' when the seasons become available to buy.
"Whew"
