The adoptable storage feature was introduced with Android Marshmallow a few years ago and it made sense for devices with low built-in storage since the microSD card acted as an additional space not just for your media but your apps as well. Unfortunately, Samsung chose not to adopt that feature on the premise that only low-storage, low-end devices would benefit. It all changes with Android Pie, though.
The adoptable storage option in Android Pie beta for Galaxy Note9
According to post in XDA Developers, the adoptable storage function will be enabled with the upcoming Pie update…