photo library

If you are looking for backup, then use time machine, which is already built into your mac. Get a USB drive around 3 times bigger than your system drive. Set up time machine to backup to it. Time machine will protect everything on your mac easily and simply.

See:

https://support.apple.com/en-gb/HT201250

Yes.

Most Simple Back Up:

Drag the Photo Library from your Pictures Folder to another Disk. This will make a copy on that disk.

Slightly more complex: Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.

Example of such apps: Chronosync - but there are many others. Search on MacUpdate or the App Store