How can I delete this extra data from my iPhone, from my current Macbook?

See the following instructions from Locate backups of your iPhone, iPad, and iPod touch - Apple Support

Delete backups

On a Mac with macOS Catalina or later:

1. Open the Finder. Click your device.
2. Under the General tab, click Manage Backups to see a list of your backups. From here, right-click on the backup that you want, then select Delete or Archive.
3. Click Delete Backup, then confirm.
4. 

You don't need to have your phone connected to the computer to see or delete backups to that computer. If you followed the instructions in my previous post and it shows no backups, that would indicate that there aren't any currently on the computer.

It's a coincidence. iPhone backups are one of the things that are included in system data but you don't appear to have any backups on your computer. Another major thing can be Time Machine backups.

System data being large is not an area that I have any particular expertise. I would recommend that you start a new post with a title something like How to reduce 122GB system data on my MacBook Pro" and include the last screenshot above in that post.

Where (on your computer or on your iPhone) does it say that you have 122GB of data stored on your computer?

The 122.65 is the storage used on your iPhone. It has nothing to do with your computer.

You can also see that (along with details) on the phone itself at Settings > General > iPhone Storage.