Open world = 1 level?/ = many levels?

hello, I am planning on making an open world game but I’m confused on how to start it in this engine. Do I use one level and build on that as much as I want, or do I separate it into many levels and place them together?
(Note: I’m not taking about adding insides of buildings/locations, I’m just asking how to set up the outside of the open world, where everything gets placed and the majority of the exploring is done.)
Any and all help is appreciated.

Please and Thank you,

For open world games, people are using Level Streaming. Basically you have 1 level and you can load chunks of other levels into the same level. Should be clear if you read the link and the subcategories, but if you have any more questions, feel free to ask <3.