.pak for Extension pack?

Hello, I am trying to create an extension pack for our client where it should contain maps, assets, and some new assets models.

I found ways to create it by using .pak files.
Are the .pak files easy to use when we already have a complete base game and want to add new customization in the future?

However I have a question regarding packaging, do we only need to package for the extension pack? (Ex: new map, assets, and clothes).
Or do we need to repackage the whole project including the base map/level?

I also have a question regarding loading the .pak files. Does the base package need to have the code/blueprint to load the .pak file or those codes can be added to the .pak?

As I wanted the client only needed to download the new extension pack while keeping the original package (base game). I also wanted to reduce the amount of packaging base game whenever there are new extension packs as I have multiple customers with different demands.