Hello Andreas.
Just a remark: there is no difference between AWS S2 data and SciHub data, except the packing/grouping of files. From an “original” S2 product, the SNAP S2 reader knows how to convert DN to reflectance (of type float), whilst if you open just a jp2 file (a single band), the SNAP JP2 reader doesn’t know how to do that and you only have DN.