Restore iMac to factory settings ?

Hi Folks,

For various reasons I need to go back to the original factory settings my iMac 27” INTEL late 2013 shipped with.

This was Mavericks.

I am currently running High Sierra but MUST go back to the original factory settings.

Perhaps best state early that I’m not by any means particularly tech savvy.

I’ve been down various dark avenues during this process and at one time came up against the “mac prohibitory symbol” which was kinda scary however I’m now back to OS High Sierra.

I had followed the following advice from Apple -

"On an Intel-based Mac, if you use Shift-Option-Command-R during startup, you may be offered the macOS that came with your Mac, or the closest version still available."

So I’ve tried re-booting in Internet Recovery Mode in the hope that I would be able to get the original OS that my iMac shipped with - MAVERICKS - reinstalled.

All went well until I chose Mavericks reinstall at which point a message stated that “You can’t upgrade this version of OS X because a newer version is installed”

I had thought that by using the Internet Recovery Mode I was starting afresh and that there would have been no previous OS in situ to prevent Mavericks installing.

My latest google searching seems to suggest perhaps “formatting” the hard drive and then once again rebooting by using Shift-Option-Command-R during startup.

Then attempt to reinstall Mavericks.

Does formatting the Hard Drive erase the existing OS and in so doing allow me to reinstate Mavericks ?

I have all my files backed up on Time Machine.

Many thanks in advance for any advice and/or assistance.

Kind regards, Garry.