In your case, it's not likely a question of IoT Core versus Greengrass. You will likely use IoT Core. It's just whether you will use Greengrass as well.
IoT Core runs in the cloud and Greengrass runs at the edge (usually). IoT Core is a cloud service but Greengrass is an edge runtime, with supporting cloud service.
You can build your device software on top of the Greengrass edge runtime. For devices with sufficient resources, Greengrass is a great way to compose and manage your device software. For your central unit, you would likely use either Greengrass or one of our IoT Device SDKs (https://docs.aws.amazon.com/iot/latest/developerguide/iot-sdks.html) to create your device software. AWS IoT Device Client (https://github.com/awslabs/aws-iot-device-client ) may also be of interest; it is a Linux reference implementation using the C++ IoT Device SDK. Greengrass is likely a good choice for your device if the device will perform edge processing (Lambdas or ML inference for example) and/or if the device acts as a hub for other IoT devices.
However you choose to compose your device software, your device will likely connect to IoT Core and publish messages to the IoT Core message broker.
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