Hi @TrioxinTrixie - I moved your post to The Sims 4 Discussion forum. (you can subscribe to your post so that you get an email when a new reply happens. Click on the Topic Options menu next to the first post & choose "Subscribe")
Thank you for the additional information, it is very helpful. I think I understand where the confusion comes from. When you start a New Game, you are starting and saving the ENTIRE world as a new save, not the particular character you were playing.
New Game (Main Menu) = Starting a new world
New Household (Manage World) = Adding new Sim Households to your existing game
Build (Manage World) = Adding new Lots to your existing game
This gives you the option to create new Sims & households, new houses & lots, and entirely new worlds. If you want all of your Sims and Houses in the same world, you can create them all in one Save. The picture you'll see under the Load Game is the last Household you were playing. But all the Sims and houses that you made in that Game are still there. If you started a new game to create a new Sim, then only the things you created in that game will be there. Here is a great step-by-step that shows how this works: https://sims4.crinrict.com/eng/2014/09/save-games/#laden
You can fix this, and get all of your buildings and Sims into one world the way you want them by saving them to your Library. You just need to decide if you want to play the original game and move your newest Sims into it, or you prefer the newest game/character - then you'll want to move all your buildings from the save that has them into your new save. Once you save to your library, then you go into your preferred game and place the missing stuff into the game.
You can save households/Sims to your Library from CAS (either from the Manage Households menu or just click on a dresser and go to CAS). To save your lots, use Build mode from Manage World. Go to Load Game, choose to open the Game that has all of your Newcrest buildings in it and then instead of loading the Sim household, open Newcrest, choose your lot, choose "Build" (bottom right of the screen) & save the building to your library.
Here's a step-by-step with pictures https://www.carls-sims-4-guide.com/forum/index.php?topic=26324.msg439569#msg439569
I hope this helps. It's not as difficult as I'm making it sound 🙂 If you have any problems or something isn't clear, please let us know. Happy Simming!