Handling ad placement as a production house?

Hello all. I don't post much, but I read the forum fairly often. I'd like to pick a few of the great minds here. For those who operate creative production houses, how exactly do you handle ad placement? Particularly when acting as an agency on behalf of the client? Do you have to register(?) with the desired media outlets? Do you take a cut of their ad buy?

The reason I ask is, im looking into starting my own production house at some point, and while I have experience at the broadcast level, I'm not exactly sure how production houses handle this.

Any help is greatly appreciated. Feel free to ramble and vent too! :)