Getting a direct ssh connection into an app is not possible. All apps run in a separate container and are isolated from the main system. Depending on the app however, most apps have a read/write folder mounted from the host system into the container to store data. For example nextcloud is setup like this. The folder is located in /home/yellowtent/appsdata/<appid>/data/ and you could rsync data into this folder with the regular ssh connection to your server. You can easily see the appid if you open either the logs viewer of the app in question and take the appid from the browers url bar. Please note that this is merely a workaround so far and it depends on the app, if it correctly will pickup the data there.