Making changes to off lot worlds - is it possible at all?

Hello creators!

I'm juuuust getting back to Sims 4 and CC content making and I've been fixating on one thing since (thanks, ADHD). I've never liked some of the shells and props around the base game worlds and Magnolia promenade. I've already tried and explored a few options to modify those worlds and make them look better, but I'm not succeeding at all so far. I know Nando has done something similar with his world overrides, but I can't for the life of me figure out how to reproduce that.

• I thought it would have been possible to swap shells for other ones in the .world files, but after a day of trying to make any change at all to the file, I haven't managed to figure out how to do this or enact any change at all like removing items.

• Alternatively, I considered overriding a few shell items I considered particularly ugly, and make them invisible, and afterwards adding items via TOOL in game. However, I don't think this is the better option, considering it depends fully on someone using a specific save file in order to make it work. Plus, shells are reused in multiple worlds, and I think even the matte background from willow creek is the same one used in Newcrest.

• It seems it's also possible to spawn in objects in the game via the festival mechanics? Like the food stands, ice rinks, and city living festival items? If that is possible, a combination of this and a few overrides of shell items would make it the easiest to install option. However, I have NO clue how to even get started on this.