Back up iPhones to external drive using Finder

There is no direct way to do this as Finder does not offer a way to re-direct backups to an external drive.

You may be able to use symlink to point the backup folder out to an external drive, but it's not a simple process.

click here ➜ Make a split library portable - Relocate iOS device backups- Apple Community

Scroll down to the section titled "Relocate iOS device backups" if you aren't directed tom it automatically.

It cannot be done via the Finder; and a symlink would be risky even if it worked. FWIW, I have had no luck with symlinks that involve ~/Library since Catalina ... and the default location of iOS backups ~/Library/Application Support/MobileSync/Backup

I suggest looking at iMazing ... it can do iPhone backups to any drive including external drives.

I backup to internal SSD, eject iPhone, then go to file location and pull the backup to external drive before deleting from internal drive. Repeating this with remaining iPhone/iPad.

Restore by reversing the steps - copy to internal SSD original backup location, open finder window and select mobile device, restore. Never change folder name or you won’t be able to restore.

A workaround is to install macOS to an external drive, boot to it, and dedicate it as iPhones' backup.

Backups are at '~/Library/Application Support/MobileSync/Backup'. I have not tested if just moving them to another disk via the Finder and, if needed, then back to the internal disk would work when restoring from such backup.