General chat is usually divided up by zone, so that if you need to find adventurers nearby to team up with for a task you can easily find them. Most questions tend to be relevant to where the players are located. New players and returning veterans do have the benefit of newcomer chat though.
If you are in a main city that general chat spans all the ‘capitals’,
Neither are exclusively realm based but you should be able to see if players are from other realms as they will have their realm name attached.
Trade works similarly to general in cities, except as far as I’m aware that is purely realm based as you can only use the services/trades of others on your specific realm.
Some realms have unofficial chat channels that are run by players but they can be controlled by any player which can cause people to be removed wihtout any just cause or silly rivalries. They used to be more popular on Classic.
I think it’s probably too busy to have on channel dedicated to everyone just chatting realm wide. That’s just a guess on my part. Realm sizes do vary.
The DF general chat is dead, dunno the reason, maybe ppl are scared to be reported and banned for no reason. The worlds chat is dead for a few years. It was very active before SL and DF expansions. Now you go to SW or the main capital city and nobody chat. The game looks dead.