So I checked out the Overlord DLC, it's alright, basically it's a bunch facilities on this one big area you drive via the firewalker hover tank. I honestly didn't think the story gives any insight on the geth, just what you already know. Here's a summary of it that I'll spoiler tag.
[spoiler]Basically Cerberus wanted to interface a geth VI with a human mind, and the only way the researchers found it possible was to use this autistic guy to be the guinee pig. It obviously doesn't work and the geth VI starts taking over shit.[/spoiler]