How can i save entire database from https://app.ninoxdb.de/...?

First .. Create a backup of the desired application .. When you mouse over the application .. You will see downward pointing arrow head.   Click that.. and select create backup. (note.. SnagIT did not capture the downward point arrow head.. so I drew one..;) 

Once you created the backup ..  Select Manage backups .. select Manual Backups.. then your desired backup.. and click Download backup.. 

If you do not see any of this.. then you are not the database owner, so contact them and they will do the needful.