Can I back up my MacBook Air and iPhone on same external hard drive?

The iPhone can be backed up to your internal drive. Then if you back up your internal drive to an external drive, the iPhone backup will be there also. You can also copy or move your iPhone backup to an external drive. That iPhone backup is located in your USER Library, Application Support, Mobile Sync, Backup folder.

But to restore from an iPhone backup, that backup needs to be in that same original folder indicated above.

Many have tried but I don't believe I have ever seen an example of successfully backing up an iPhone directly to an external drive and have that backup be sustained through a MacOS update and then be used to restore an iPhone. Such an arrangement is not supported by Apple, having such a backup is worthless unless you KNOW it will work when you need it.

You might consider using iCloud to back up your iPhone. I think that is the most common method and if you wander into an Apple Store and informally survey customers who are purchasing new iPhones, the vast majority of them simply restore their iCloud iPhone backup to the new iPhone to get migrated over and start using the new iPhone.