flatpak can operate in system-wide or per-user mode. The system-wide data is located in $prefix/var/lib/flatpak/, and the per-user data is in $HOME/.local/share/flatpak/.
In addition to the default installation locations, more system-wide installations can be defined via configuration files /etc/flatpak/installations.d/, which must have the .conf extension and follow the format described below.
All the configuration for the the installation location with name NAME is contained in the [Installation "NAME"] group.
The following keys are recognized:
[Installation "extra"] Path=/location/of/sdcard DisplayName=Extra Installation StorageType=sdcard