No source needed. All plug ins work this way - they are just a hybrid with a larger battery and ability to plug in to charge. Think about it! It is simple. But there are diagrams out there of the architecture.
Yes, most of the added battery cells in the PHEV are in two locations, but they are treated as one large battery (just as the many cells that make up the hybrid battery are treated as one battery). It would be highly inefficient to have one battery for the hybrid mode and one for plug in EV miles.
In the North American Niros, there is a 12 V lithium battery attached to the 360 V traction battery under the rear seat, but obviously electrically separated. In 2017 Ioniq/Niro models other than North America came with a 12 V lead acid battery in the right rear of the cargo hold. All the PHEV's come with the same 12 V lead acid battery in the same location. I don't know why actually. The reason given for the 12 V lithium battery was to save 40 pounds of weight. The PHEV traction battery is 480 volts I believe as an aside.
2018 Kia LX HEV Metal Stream with Advanced Tech