Or will all connections be automatically closed while the App closes?
No one knows an answer, please? 
Maybe you can check this by looking at the open connections of the database while shutting down the app without closing the connection. Normally the database engine should check if a connection is alive and if not close it by itself otherwise there is a huge security risk. However there are settings in the db-engine to keep connections alive for a settable time, which should be as short as possible.
When the db object goes out of scope it will automatically close.
Thank you Andre and Bob.
I asked because i have open connections to 7 different mySQL and MSSQL Databases. And sometimes, closing the app takes a while.
In my Closing Timer, i had the following code multiple times to close each connection:
rs = OTRS_DB.SQLSelect("SELECT 1 FROM DUAL")
If rs<>Nil Then OTRS_DB.Close