I am trying to understand how to create a scaffolding, its template etc, in AEM 6.5, since in the restructuring document it is mentioned, that it has to be re-developed instead of migrated [6.3-->6.5].
Scaffolding is classic UI enabled feature and therefore it will take a lot of effort to build afresh in AEM 6.5 where all things like custom template, form elements need be defined. If you can stick to classic UI then that's the way to go.
Did you check if content fragment as an alternative. It sort of does the same thing but saves the content in dam hierarchy.