Best Way To Make Open World?

Hi,
I’m thinking of buying an asset that helps with RPG and includes networking, but the network only supports one scene which would mean everything would have to be open world. Do you know the best way to make an open world in Unity, so that it doesn’t lag?

I heard of occlusion culling, I don’t know if that would work? Maybe the game would only render or load objects/terrains in a scene that is within reach, if that makes sense. Is it possible to get something to work with the editor too? If I have a gigantic scene full of like 20 different maps then I think it would be better if there was a way to not make the editor render the entire scene otherwise I’d get severe lag just trying to develop the game.

Thanks for your ideas!

You can buy Invecotor melee/shooter combat template, but i’m not sure it supports multiplayer.
For terrain creating you have Gaia, Terrain Composer and Relief terrain pack.
For 3d models you can search for publisher in aseet store: noirfx and Manufactura K4.
Invector asset supports many asset like dialogue system and it have got many addons like swimming, rpg addon, car addon and more…