Configuration

AppFlowy can be ran in cloud mode (using AppFlowy servers), on your own server (self-hosted cloud) or locally. Currently I use it only locally as I don't need the collaboration features, but with a multi-device synchronization via Nextcloud. This works well.
Make sure AppFlowy is actually in local mode, as this seems to have an impact on the data path. On 0.9.4, you need to go to Settings -> Cloud Settings and choose "Local" for Cloud server.
AppFlowy is a Flutter application and as such does not respect correctly font sizing (on KDE). The only workaround I found so far is to launch the application with the GDK_SCALE environment variable (so for instance env GDK_SCALE=2 /usr/local/opt/AppFlowy-0.10.3-linux-x86_64.AppImage). This works but unfortunately GDK_SCALE accepts only integers so it won't work with GDK_SCALE=1.7 which would have been ideal.

Upgrading

Upgrading AppFlowy can be very dangerous. Make sure you backup the whole database (/home/elvanor/life/app-data/appflowy/) before attempting an upgrade.

In local mode (not sure about the others), the data path location (let's call it APP_FLOWY_DATA_PATH) is by default ~/.local/share/AppFlowy/data. However, you can change it in the file ~/.local/share/AppFlowy/shared_preferences.json, by using the key "flutter.io.appflowy.appflowy_flutter.path_location". For instance:

"flutter.io.appflowy.appflowy_flutter.path_location":"/home/elvanor/life/app-data/appflowy/data"

Note that this folder contains another directory with a random number as name (480979045434003456 for instance), which seems to correspond to a session id. It contains the database file, see below.
As it can be difficult to change the location of data (do not try to move files inside the data folder!), you can use the Import feature from AppFlowy (Settings -> Manage data -> Import data). Choose the base data directory. It will work correctly but may require a restart of AppFlowy to be visible, and as it creates a subpage, you may need to move the imported page(s) around.

It's not clear which files actually host the contents of your documents. APP_FLOWY_DATA_PATH/480979045434003456/flowy-database.db may be an important file, but APP_FLOWY_DATA_PATH/cache.db also contains critical data (last user session information, which is needed for AppFlowy to properly start). Note that those two files are Sqlite databases.