We are setting up a database using Monday.com. Is it in a relational database format? Where is the data stored? Whilst changes can be made using one board, across others that share the information, which is (or where is) the ‘master’ file?
Hey @Nicky.Day ~ Welcome to the Community! 
If you are asking where the data officially lives when utilizing The Mirror Column, then this would be the board that the column is originally created in! However, since the mirror of this column is directly connected to that board, changes made in one column will be reflected in the other as well.
Check out this article for more details on linking boards together in general: The Connect Boards Column
If you’re looking for more information on our data security and where your files are hosted, you can refer to this article: Is My Data Secure?
Does this answer your question? Remember to mark as solved if this answers your question 
Cheers,
Jenna