Dear Dieter,
"sheet rolling, aluminium" is a service that does not include the aluminium that will be rolled. You can often find infos for things like this inside the description of the product flow:
This is delivering the service of 'sheet rolling, aluminium'. The activity consists in the rolling of aluminium aiming to produce sheet. The sheet thickness ranges from 0.2 to 6 mm. The material being rolled is not included in the dataset, thus needs to be added separately. Consequently, for each kg of product rolled, it is necessary to add a kg of base material. It can be used in the following applications and/or processes: aerospace industry for the skins of planes, transportation industry in the auto body sheet, packaging and construction industry in building facades.
The advantage of this modeling by ecoinvent is that you can use that service for different types of input materials without having a separate process for each. The same goes for steel or copper. You add the service and the raw material with the same amounts to your process. You can spot these kind of service flows by having the verb for the service inside the names like moulding, rolling, winding etc. But best ist to always check the flow description.
Here is also an example how it looks in one of the ecoinvent processes:

Best wishes,
Conrad
PS: I don't remember if the flow description was already part of version 3.9.1. We have it for sure in 3.10 and 3.11 and it was not there in 3.8 and earlier.