In our factory environment, we have multiple IoT Devices (Machines, Sensors, etc.) that send different metrics. Also among machines and sensors, these types of metrics can differ. I want a system where the digital twins for these devices are created automatically based on the metrics they send. Do I now have to create a unique model for each device or is it somehow possible to use a "general" model template, where twins can have the same model but different properties?
All twins of the same model share the same properties, so in your case, you might need to create a model per device type. If the telemetry differs per device for the same device type, you could consider making a model for that device type with a superset of all the properties.
Azure Digital Twins Models also support inheritance, which could be useful when creating a model per device type.
There is currently no support for model-less twins or creating them based on the input metrics.