Question Where is localization package doing its internal StringTableCollection loading?

Its done inside of LoadTableOperation. You cant modify this unless you just change the code in the package. You can move the package out of the package cache folder and into the Packages folder of your project, it can then be modified.
I would not recommend modifying this though, any issues you are having are likely to be with Addressables, not this operation.
1.4.2 should be out next week, it has features for providing table patchers and providers.

 

Have you managed to get the content update workflow working?
https://docs.unity3d.com/Packages/com.unity.addressables@1.20/manual/ContentUpdateWorkflow.html

Try doing it with something very simple first, such as a Texture with no localization package involved. Update the texture and rebuild, see if it updates on the device. Make sure thats all working fine first.

 

Try using the Addressables Editor hosting to test. I have used this and its worked fine for me. If that works but your server does not then you are 1 step closer to figuring it out.