|Question as written is hard to understand.
Presumably you have a delivery that looks like the following
And you are loading Extended dll dynamically.
In that case Extended dll should be built using Base.dll.
There are two possibilities there.
1. You have already referred to something in App.exe which is in Base.dll. And thus Base.dll will already be loaded into the process (AppDomain) space
2. You have not referred to Base.dll. In that case the .Net load process will attempt to search for Base.dll. Presuming it is not in GAC then it will search in the App.exe directory and thus it will find it. And load it.
The above presumes you haven't messed with anything including setting up your own AppDomains.