Can someone help us to develop a custom Box app?

They have a getting started guide here https://developer.box.com/guides/getting-started/first-application/

How is the file uploaded into box? Is that an event that happens every time at the same time and is the trigger for the database update or is the file uploaded once and the update to the database happens regardless of whether that file is touched or not? Also, what are you using to update the database? Is it a custom tool or a standard interface?

Lastly, have you looked at Relay which is the BOX workflow tool and see if that gets you all or part of the way?

Use the Box CLI on an internal server.  Get your external file with the CLI and save it local for processing.  Then use other tools to import the data to your DB.

I personally use PowerShell.