@watcherman Sorry I was being rude, you didn’t deserve that.
But my point still stands, that there are configuration options that can be set in order to keep areas loaded. The last part was merely an addendum, you are of course free to change those options.
The whole world cannot be kept loaded, as the amount of chunks that can generate generally exceeds the amount of ram. The same goes for the overworld.
If you want specific areas to be loaded, as opposed to worlds,
You need to set those configuration options to prevent the worlds unloading (assuming they work, if they don’t file a bug report)
You need something (There are several mods, maybe even some plugins) that can provide chunk loading capability for a given area. If something is being explicitly chunk loaded, the worlds probably shouldn’t unload anyway regardless of config option, but they won’t hurt.