How select which drives to backup?

To be specific, nothing is backed up until you do so with Time Machine, using the information o the link above. Also, be sure to back up to an external disk which is dedicated solely to Time Machine backups.

Jhturley wrote:

I have a new Mac Mini running Ventura. I don't know what is being backed up.

click Time Machine icon in the upper right menu bar> Open Settings >Options

Time machine backs up your User data, you can exclude files/folders here

—Back up your Mac Back up your Mac with Time Machine - Apple Support

When doing something new, red the instructions and, if necessary, the Help menu's information, then carefully understand and follow the instructions. People who get into trouble are often those who do not follow this advice.