Applications: How does OS X Know Which App to Reference a File to?

I just got my new Macbook and while installing applications, a question I've had for a while came up again.
How does OS X know to reference applications to certain file types?
For example. I have a .avi I downloaded to watch, and downloaded VLC to watch it. I open the .dmg and drag/drop VLC to my Applications folder. Instantly, VLC appears in the "Open With" list for my .avi. How does it know to list it?
My theories are:
1. When dragging/dropping VLC, it's not just copying files, it's also doing some background stuff, including VLC telling OS X that it should be referenced for .avi files
2. When I "Open With" a file, OS X looks at apps in my Applications folder and detects applicable ones in some manner.
3. It's Voodoo magic.
How does it work? I hope my question makes sense.
Thanks!

From what I understand, the second the .dmg is opened and VLC is seen by OS X as an application, Launch Services asks VLC what filetypes to associate it with, and adds that to the database? Then when I try to "open with" a .avi, it sees that .avi is associated with QuickTime, and now VLC too.
that's right. inside the VLC application bundle there is a file called info.plist which contains among other things the info about which files VLC can handle. that file is consulted by the Launch services once it becomes aware of VLC being present in the system and that info is recorded in the launch services database. That's how it works with all applications. you can view this file if you want. control-click on VLC in finder and select "show package contents". in the new finder window that opens up go to Contens folder and quicklook the file info.plist.
Is this correct?
Thanks.

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