Getting into this for the first time today. Is this game legit cross play, like I can trade with pc players on my ps5 or do I have to purchase this separately on all platforms like d2r?

Its legit crossplay. You just need their bnet accounts

Just make sure you have not opted out of crossplay in your settings. Also a few times I couldn't play with my xbox buddies when blizzard disabled crossplay but that was only a few days out of multiple seasons.