How to access .dll or .so files in an android project

I have a folder with a lot of .so files which I'm trying to use while my code is running. I place them in a folder called "External" and then I try to access them like this:

[DllImport("External\\libvocon_ext_heap.so", EntryPoint = "ph_CreateWin32PrivateHeap", CallingConvention = CallingConvention.Cdecl)]
public static extern PH_ERROR ph_CreateWin32PrivateHeap(ref LH_HEAP_INTERFACE pHeapInterface, out IntPtr ppHeapInst);

I've also tried to place the files in the Assets folder, but i still get the same System.DllNotFoundException