Want to fully understand how it works. Been googling for a whole day but still couldn't find the answer I was looking for. If anyone has the blizzard website(not wowhead or any wow wiki stuff bc Ive read all of it) for full explanation and special exceptions with sharding, please share.

I read that rp servers dont share with other realms or get "sharded" except when it's to to much or when events happen.

I want to understand why am I seeing WrA[Rp-realm] Horde players in the open world with warmode on(I am ally Moonguard[RP-realm]). Can I go rek some WrA horde players in OG while staying on Moonguard ? Does sharding affect the whole zones ? How do I know which zones/Areas will my pvp gear change to its pvp ilvl bc sometimes my ilvl just changed in a non-pvp- freeforall zones. I have pve gear and pvp gear and if I am not doing the world pvp quest, I'll have my pve gear on but sometimes while lvling I got attacked by some horde players and couldnt change gear in combat. I didn't upgrade my pvp gear ilvl so I don't want to use it in pve. But it seems like I'll need to upgrade all my pvp gear to max and never change to my pve gear in the openworld, only in raids delves and dungeons.
Thank you for the responses