They plumb into the factory fill neck. Most use a small pump that taps into your fuel gauge wiring, so when your main tank reaches 1/4, the pump kicks on and fills the main tank until the gauge reads 3/4, then it shuts off.
I have the new Titan in-bed tank along with the Titan underbed tank, so I can put a total of 112 gallons on the truck if I want. The in-bed tank transfers into the main tank much like the Transfer Flow, except mine is just gravity fed, no pump on it. It does have an electric solenoid I can open and close with a switch in the cab, so I can fill my main as needed. I liked the Titan in-bed because it doesn't have to mount permanently. There are some brackets you mount in the bed, then the tank just slips in and out of them easily. So I leave the tank out all the time and just put it in when I'm taking long road trips or towing a lot. The plastic tank is light enough, when it's empty, I can install/remove by myself in about five minutes. It can also be used as an auxiliary tank, there is a place to put a transfer pump in it to fill equipment or other trucks with if you need...