This should help get you started. As far as your other questions are concerned, I can only speak Horde side. Numbers are objectively down compared to the tail end of Legion, but probably the highest I’ve seen since people hit level cap in BfA. The Valley of Honor, Silvermoon City are chock full of people looking for walk up RP, especially during peak times.
CrossRP was changed. The old method seemed to use the Communities feature in a way Blizzard felt left it open to exploit, so they patched it and in effect got rid of it. They later released the Elixir of Tongues, which only translates Orcish/Common in /say. The CrossRP addon lives on and when used in conjunction with the potion, allows you to emote, refresh the potion automatically, whisper (you need to be Battlenet friends), and link parties for larger events.