My sims are all in separate worlds/loads

[Simplications]

I want my sims to be able to communicate with one another without being in one household but it seems I can only have one household per save?

My past sims do not exist in the new world i create, none are in the same world at all

[Phantomlover1717]

You can have them all in one world/save. Just open the save, go to the world view map and let them all move in.

You can either find them in the library or you might have to go to the old save to add them to the library yourself.

[Simplications]

What about the lots I built? How do I love those over?

[jpkarlsen]

Save the lots to your Library then place them in the other savegame.

[Phantomlover1717]

Move the lots and all you want to save to the library. You can place them back in the new world.
You can always keep the old save if you want to go back and check something.