How to Alter Shell Builds In A World?

Sims4Studio. Under the Objects section click Override, check the box labeled debug, find your building that you want to replace and make your package, then just follow any object tutorial to replace the buildings. Some buildings share textures or meshes so you will need to find their counterpart to replace as well, or dig into the Warehouse and manually switch the reference to a new texture so you don't have to do that (would be under the Model file).

Wow! Thank you for replying! Umm....honestly, you lost me at “object tutorial”. ?

I mean literally any object tutorial to show you how to use S4S and adjust objects, lol. This one is a beginners tutorial on objects. You're ignoring the cloning part and instead choosing Override. Not sure where you are getting your objects or if you are making them from scratch, but just follow along. There are other tutorials covering objects out there that might help.
https://sims4studio.com/thread/856/finish-object-tutorial-absolute-beginners