How do I save a world for future games?

You could put a in a new tray folder and download the lots in there. Then you can quickly place them in any save that you would like to use them in.

I have a separate tray folder for each world. I have a lot of builds saved and I needed a way to organize them.

I built a save earlier with specific lots placed with the intention of using that save as a starting point for future stories. Then that save got corrupted and I was irritated over the time I spent setting it up. That is when I decided to go modular instead. Each time I start the game I decide on which world I would like to modify that time and put in that tray folder. After using this system for years I have different styles for different saves, plenty of community lots that suits the world. Any work I put into a tray can be accessed by all my different saves.

Also, I like this kind of stuff. Organizing things with a cup of tea is a way for my mind to relax, otherwise I would probably not have chosen this setup.