GeoProcessing Ignorance

I have never used GP before so please excuse my ignorance.

Let's say that I wish to develop a custom GP tool which uses ArcObjects
and I wish to use 64-bits. My particular problem relates to a data
conversion issue and it is limited by available virtual memory so x64
seems like a nice option. I am assuming that this will use the new
x64 support for background GP in 10.1 ArcMap.

How do I determine which ArcObject libraries that I can access?
Is it the libraries which say that they are available for ArcGIS Server?

It appears to me that the underlying machine uses RuntimeLocalServer
which I believe is the new ArcGIS Runtime.

What I have is a custom desktop product (x86) and a custom server
product (x64). This includes custom feature classes for which I have
an x86 and an x64 version.

Do I need to install the x64 custom feature classes on desktop?
Does RuntimeLocalServer even use them? I ask because my understanding
was that customization was very limited in runtime.

The thing that I am trying to understand is the execution context for GP.
I understand what happens in ArcMap and I understand what happens in
ArcGIS Server. Is there a notion of an extension spinning up?