How do scripts work in relation to the DLCs?

[Parmie]

Hello people! I am very new to DS modding, and I am currently in the phase where I'm trying to get a grasp on how the game works. One thing I've ran into is a lot of confusion about how DLC scripts work. I will try to explain some of my confusions and I hope someone might clarify them.

I know that the game files have 4 different scripts folders, one for the base game and 3 for the DLCs. Some of  the .lua files seem to have versions in DLC folders with added or changed functionality, but definitely not all. To my current understanding, which folder is used is based on which DLC world you are currently in, which I noticed when debug keys didn't always do the same things in the different DLC worlds. I also wonder how this works with DLC compatibility? Because when you create a new world you can choose to get shipwrecked and/or hamlet compatibility, so clearly the game somehow 'knows' to load certain files from different folders, because you can craft items from other DLCs in another world and take DLC items with you. And what if you need multiple files of the same name in different folders?

Thank you in advance!