How does patch system internally works?

Question

Hello,

I've been trying to find an answer to this question over any of the documentation pages but it seems like I can't find any solution.

I'm wondering how does the patch system internally works and deliver the updates to the users so my questions are:

If I publish a game which game size for needed packages is 10 GB and then update the game, modifying only a small amount of the package in order to make bug fixing, will the user have to download again the whole 10GB in order to update the game or does it only download the diffs?

I understand that if I add new packages the user will need to download them, I'm curious of what happens when the already downloaded packages are updated.

Thank you in advance for your time and hope someone can offer me some clarity regarding this issue.