Both the Meter and the app store data.
The Meter automatically maintains a backup of the most recent 36 days of readings, so while you are away from the Meter, it will continue to record the conditions. When the Meter reaches 36 days, it starts to overwrite the oldest data points.
Once the data is picked up by the app, then that data is stored indefinitely by the app on your mobile device.
Similarly, once the data is picked up by a SwitchBot Hub (Plus/Mini) (when a Hub is used in conjunction with the Meter), then the data is stored indefinitely in the SwitchBot cloud.