Interactions between code in two unnamed modules

Premise: I have had 2 classes loaded by different class loaders - one from the app classloader and the other, a custom class loader. The class loaded by the custom class loader is trying to reflectively access a private field in the class loaded via the appclass loader. In this case, I get a runtime error along the lines of "Failed to access class A from class B; A is in unnamed module of loader 'app' and B is in unnamed module of loader 'Custom'. The error goes away after adding an '--add-opens java.base/java.lang=ALL-UNNAMED'

Questions : I understand that there can be up to one unnamed module per classloader in the VM.

1. What part of the JLS or JDK implementation specific to hotspot VM talks about interactions between two unnamed modules across loaders ? I can see a lot of unnamed to named module interactions etc in the document, but not between two unnamed.

2. I understand why add-opens is solving the issue in general( the code is being invoked reflectively and end calls into JDK happen via java.lang.reflect APIs?). However, unclear again as to how the add-opens works across loaders - does --add-opens=ALL-UNNAMED expected to open the packages in the specified module to all unnamed modules across loaders in the VM or only to that loader?

Using Java 17 + hotspot VM.