First and foremost: All data that is entered in the app never leaved your phone and tablet.
This includes all information that is provided by you, but also all data that is downloaded from your FHEM server.
To make it explicit: This of course also includes username, password and connection url of your FHEM instance. However, this also includes the xmllist data, which is downloaded from the server.
There is the option to send a bug report to the developer. This will include data about what has been downloaded and might also include the connection URL. However, this will never be sent without your consent and never without your manual interaction.
AndFHEM also collects telemetry data to improve the app in general. This includes data about app crashes, which are sent to the developer.