Can I backup my iPhone on an external drive?

You can backup the iPhone to a computer, and then move that backup to the external drive, but there is no way to backup directly from the iPhone to an external drive.

The most you can do, is manually move files and photos to the external drive.

Same general way, you create a sym-link to the external drive for iTunes to use as the backup location. Details here under "Relocate iOS device backups": Make a split library portable - Apple Community

But this is not backing up directly from the iPhone to an external drive, i.e plugging the drive into the iPhone and generating a backup. Its backing up to iTunes. Where iTunes then places that backup is a different thing all together.

Yes you can if you have a Mac!

There is an article on it here: https://lifehacker.com/back-up-your-iphone-to-an-external-hard-drive-with-a-te-1762962368

It may be possible with Windows 10 iTunes but I don't know how you would do that....