From https://learn.microsoft.com/en-us/azure/iot-develop/concepts-digital-twin#device-twins-and-digital-twins, it says "Along with a digital twin, Azure IoT Hub also maintains a device twin for every connected device. A device twin is similar to a digital twin in that it's a representation of a device's properties. An IoT hub initializes a digital twin and a device twin the first time an IoT Plug and Play device is provisioned".
I'm trying to understand where the digital twins and the device twins are kept. Are they part of the Azure IoT Hub? There's another Azure service called Azure Digital Twins (https://azure.microsoft.com/en-us/products/digital-twins/). Is that where the IoT digital twins are kept? Even though I can see some devices and device twins in my Azure IoT Hub, I don't see any digital twins in Azure Digital Twins.
Your Azure IoT Hub maintains both the device twin and the digital twin - as the article you referenced describes, they use different JSON representations of the device state. There are separate APIs for each.
Azure Digital Twins is a separate Azure service that extends the concept of the digital twins maintained by IoT Hub. In ADT, the model can represent complex clients, such as smart buildings.