With Marshmallow, Nougat and above you first have to format the SD card as Adaptable Storage (AKA Internal Storage). Not every phone will support Adaptable Storage (the G4 does), some manufacturers might remove the option. Slow SD cards might not be compatible.
There should be a notification when you first put the SD card in to set it up, or go to Settings -> Storage -> Select Card -> Menu -> Format As Internal (might vary by phone). The Portable option is for using it as regular storage, Internal is Adaptable Storage and will combine the SD card with your phones storage
Then you can go into Settings -> Apps and choose to move apps that support being moved (not all do).
The SD card is erased and formatted, then encrypted when used as adaptable storage, backup important data on it first.