Hello, I wonder if it is possible to transfer backpack data from one player's UUID to another player's UUID - may it be through commands or server files?

You can either edit it in the database directly. Or use the backpack and restore functionality (/backpack backup and /backpack restore).

You can either edit it in the database directly. Or use the backpack and restore functionality (/backpack backup and /backpack restore).

May I know where in the server files could I find the database?

It's the backpack.db file. You can open it with https://sqlitebrowser.org/ please only modify that file while your server is turned off.