Where can I find the files/inner workings of the intelligent search?

Also, I see that an overlay system still exists. How is an overlay launched and what files are involved in it?

I'm considering modifying to an overlay for the "Add Block" functionality. I'd like it to work like this:
1. Click the + Button - the places a block can be added to are highlighted and contain an "add content here" link.
2. When clicked an "Add Block" overlay opens up, functioning mostly like in previous versions.