Q: Can I backup only selected file(s)to iCloud eg. iBank?

No, you can't choose what specific data you can backup - it's a matter or all or nothing.

iCloud doesn't store or sync iBank data.

You can backup an iOS device (iPhone/iPad/iPod Touch) to iCloud. You cannot backup a Mac to iCloud.

You can sync files from a Mac to iCloud. If you go to Apple menu > System Preferences > iCloud and put a check mark in front of iCloud Drive you can then sync specific files to iCloud by putting them in the iCloud Drive folder on your Mac (Finder Go menu > iCloud Drive). Note that doing so is not a backup as any changes made to the file on your Mac (including errors or accidental deletions) will be synced to iCloud Drive.

Note that Banktivity (current name for iBank) has tested using iCloud Drive for it's files and that they note there is a potential risk if 2 users update at the same time: https://www.iggsoftware.com/support/articles/?s=icloud+drive

They also have their own cloud system: https://www.iggsoftware.com/support/articles/ibank-5/how-do-i-use-ibank-cloud-sy nc-to-share-documents-between-my-devices…