If I backed up my iphone on HDD, would the data stored on mac by any chance?

Hello honeybadgerlol.

Thank you for using Apple Support Communities. We want to make sure we're on the same page and understanding your issue/question correctly. That way we can do our best to assist you and provide you the correct information you need. It sounds like you created an iPhone backup to an external hard drive and would like to know if that information stays there or if it stays on your Mac.

There is a high possibility that your backup is saved on your external hard drive and not on your Mac. To confirm, it would be best to attempt to locate your backup. The following resource can show you how to locate your backups:

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

The resource we shared also mentions the following: "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."

So if you confirm your backup file is saved in your external hard drive, it would be best to relocate that file back to your Mac.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

We hope this information is useful. Cheers.