Hi @nico888 ,
The Power Apps SharePoint connector does not cater for creating document content in a library - you can browse document libraries in Power Apps by connecting to a library as a SharePoint list data source, but this is simply because the frontend to every document library is actually a special kind of list - so all you're actually seeing in Power Apps are document properties.
The actual document content is stored in the backend and is not directly accessible through the connector, so you can neither download nor upload document content using this connector.
As @WarrenBelz noted, the easiest way you can achieve this is to use the SharePoint connector available in Power Automate. You can use the "Add picture" or Attachment control to get document contents from a device into Power Apps - however you need to pass that content data to Power Automate in order save it to a document library using the Create File method.
At the far end of the solution spectrum, I guess you could possibly create your own custom connector for SharePoint Online and include a method that allows you to post document content into the SPO REST API - you could connect this connector to your Power App directly and use it - but then you're basically doing a tonne of work just to avoid something that would take less than two minutes in Power Automate, which is kind of like saying "I don't want to walk to the bus stop to catch the bus, so I think I'll build a car instead..."
Kind regards,
RT