When back up iPhones to MBP, where stored? Move to external HD?

This article warns that you should not move your Backups. It may be overly cautious, but I would no longer expect to restore from them if I moved them.

To avoid ruining any of your backup files, here's what you should know:

• • The Finder and iTunes save backups to a Backup folder. The location of the Backup folder varies by operating system. Though you can copy the Backup folder, you should never move it to a different folder, external drive, or network drive.
  • Don't edit, relocate, rename, or extract content from your backup files, because these actions might ruin the files. For example, to restore from a relocated backup file, you need to move it back or the file won't work. Backups serve as a safe copy of your entire device. So while you might be able to view or access a backup file's contents, the contents usually aren't stored in a format that you can read.

Yes you can. But if you need to restore from those backups you have to move them back to the exact directory you moved them out of. They are in your User Library => Application Support => MobileSync

Thanks. Of course back up is if need to restore. Does mean that iPhone back ups will take up space.

Yes, everything takes up space. Decide how many backups you need for the iPhone and delete the older ones. See the linked document for the method to do that.

Anywhere I can read what's in "other"?

There are about a thousand posts, here, on that topic.

Search for “what is other”. A web search may produce better results than searching, here.

Another source of Other that often isn’t mentioned is other user accounts. If you have multiple users on the Mac, you can’t see their files, so they become “Other.”