The value of the Shared Native Code model is the sum of the virtual memory sizes of all loaded .so files. It is calculated by reading /proc/self/maps.
The actionable breakdown of the shared native code model is a breakdown by library name. Unfortunately, due to technical limitations, this does not include information about what caused a library to be loaded, whether the library was loaded by the app or the platform, the library dependency graph, or what is causing a library to remain loaded. Individual .so files can be further broken down using tools such as readelf.
For example, for an application AmmTest.apk that includes libammtestjni.so as a native library that loads 36 KB worth of memory regions, BaseClassLoader will be shown with library /data/app/com.android.amm.test-_uHI4CJWpeoztbjN6Tr-Nw==/lib/arm64/libammtestjni.so taking up 36 KB.